Dual-energy X-ray absorptiometry in small subjects: influence of dual-energy X-ray equipment on assessment of mineralization and body composition in newborn piglets

Anthropometry-based prediction of body composition in early infancy compared to air-displacement plethysmography

BACKGROUND

Anthropometry-based equations are commonly used to estimate infant body composition. However, existing equations were designed for newborns or adolescents. We aimed to (a) derive new prediction equations in infancy against air-displacement plethysmography (ADP-PEA Pod) as the criterion, (b) validate the newly developed equations in an independent infant cohort and (c) compare them with published equations (Slaughter-1988, Aris-2013, Catalano-1995).

METHODS

Cambridge Baby Growth Study (CBGS), UK, had anthropometry data at 6 weeks (N = 55) and 3 months (N = 64), including skinfold thicknesses (SFT) at four sites (triceps, subscapular, quadriceps and flank) and ADP-derived total body fat mass (FM) and fat-free mass (FFM). Prediction equations for FM and FFM were developed in CBGS using linear regression models and were validated in Sophia Pluto cohort, the Netherlands, (N = 571 and N = 447 aged 3 and 6 months, respectively) using Bland-Altman analyses to assess bias and 95% limits of agreement (LOA).

RESULTS

CBGS equations consisted of sex, age, weight, length and SFT from three sites and explained 65% of the variance in FM and 79% in FFM. In Sophia Pluto, these equations showed smaller mean bias than the three published equations in estimating FM: mean bias (LOA) 0.008 (-0.489, 0.505) kg at 3 months and 0.084 (-0.545, 0.713) kg at 6 months. Mean bias in estimating FFM was 0.099 (-0.394, 0.592) kg at 3 months and -0.021 (-0.663, 0.621) kg at 6 months.

CONCLUSIONS

CBGS prediction equations for infant FM and FFM showed better validity in an independent cohort at ages 3 and 6 months than existing equations.

The effect of excessive fat tissue on the measure of bone mineral density by dual X-ray absorptiometry: the impact of substantial weight loss following sleeve gastrectomy

PURPOSE

Dual-energy X-ray absorptiometry (DXA) is used in clinical routine to determine areal bone mineral density (aBMD). However, it is not clear whether excessive fat mass or substantial weight loss modify the aBMD measurements. The aim of this study was to evaluate the effect of soft tissue composition on aBMD measured by DXA using a clinical model (i.e. sleeve gastrectomy: SG) that induces substantial body weight loss.

METHODS

Areal bone mineral density and body composition (fat mass: FM and lean tissue mass: LTM) were determined by DXA in 41 obese patients (33 women, 80.5%) just before SG and 1 month later.

RESULTS

One month after SG, mean weight loss was -9.8 ± 2.6 kg, with a significant decrease in LTM and FM (kg) ranging from -7.3% to -9.5%. The relative variation in aBMD was increased at the lumbar spine (2.45 ± 3.44%) and decreased at the hip (-1.47 ± 2.28%), whereas no variation was observed for the whole body and radius. The variation in aBMD at the lumbar spine was inversely correlated with variations in weight, whole-body FM and trunk FM, but not LTM.

CONCLUSION

This study shows evidence of a potential effect of body composition, particularly FM, on aBMD. However, given the modest change in aBMD, which was close to the precision error of aBMD measurements, it appears that significant weight loss does not have a clinically significant impact on the evaluation of aBMD using DXA.

A comparison of body composition estimates using dual-energy X-ray absorptiometry and air-displacement plethysmography in South African neonates

BACKGROUND/OBJECTIVES

Neonatal body composition is an important predictor of future metabolic risk; however, the comparability of objective assessment techniques, particularly in African populations undergoing rapid health transition, is not known. This paper compares body composition estimates by air-displacement plethysmography (ADP) and dual-energy X-ray absorptiometry (DXA) in South African neonates.

SUBJECTS/METHODS

Fat mass, fat-free mass and body fat percentage (%fat) estimates by ADP and DXA were compared in 88 urban, black South African neonates. The level of agreement between the techniques was assessed using Bland-Altman analyses.

RESULTS

Significant correlations were observed between ADP and DXA measurements of fat mass (r=0.766), fat-free mass (r=0.942) and %fat (r=0.630); however, ADP estimates of fat mass (408±172 g vs 337±165 g; P<0.001) and %fat (12.9±4.4% vs 9.9±4%; P<0.001) were significantly higher and fat-free mass (2681±348 g vs 2969±375 g; P<0.001) significantly lower than those by DXA. Fat-free mass estimates showed greater consistency in the level of agreement between the techniques compared with fat and %fat estimates where the differences between methods were less predictable.

CONCLUSION

Although ADP and DXA body composition estimates are highly correlated in neonates, significant differences are observed between the techniques. This is particularly relevant for fat mass and %fat estimates, where differences are highly variable between methods. Further investigation is needed to minimise inter-method differences to ensure accurate and comparable assessment of body composition at birth and across longitudinal study follow-up.

Effects of motor physical therapy on bone mineralization in premature infants: a randomized controlled study

OBJECTIVE

To study the effect of physical therapy on bone mineralization, weight gain and growth in preterm infants.

METHOD

After fulfilling the inclusion criteria, preterm infants were matched for gestational age and birth weight and then randomly assigned to the physiotherapy group (PG, n=15) and control group (CG, n=14). The PG received motor physical therapy for 15 min daily, 5 times per week until hospital discharge. Bone mineralization was measured by total body dual energy X-ray beam absorptiometry (DEXA) at the onset and end of the study. Statistical analysis was realized by ANCOVA and linear correlation tests.

RESULT

The physical therapy group (PG) presented greater body weight gain per day (27.4+/-2.4 vs 21.01+/-4.4 g, P<0.001) and length (1.3+/-0.3 vs 0.8+/-0.2 cm week(-1), P<0.001) than did the control group (CG). Body composition values verified by DEXA were greater for the PG. The mean gain in bone mineral content (BMC) (mg) was greater in the PG (434+/-247.5 vs -8.9+/-11.4, P<0.001), as was the mean bone mineral density (BMD) gain (mg cm(-2)) (8.4+/-5.6 vs -3.1+/-5.5, P<0.001). The gain in bone area (BA,cm(2)) was 10.3+/-5 in the PG vs 1.5 +/-2 in the CG (P<0.001). The gain in lean mass (LM) (g) in the PG was also greater than in the CG (271.1+/-21.4 vs 109.1+/-1.0, P<0.009). The fat mass (g) was similar between the groups (P=0.432).

CONCLUSION

These results showed that physiotherapy in preterm infants produced greater gains in growth, body weight, BMC, BMD, BA and LM.

Regulation of mammalian ciliary beating

Recent advances in our understanding of the structure-function relationship of motile cilia with the 9 + 2 microtubular arrangement have helped explain some of the mechanisms of ciliary beat regulation by intracellular second messengers. These second messengers include cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) as well as calcium and pH. cAMP activates protein kinase A (PKA), which is localized to the axoneme. The cAMP-dependent phosphorylation of PKA's main target, originally described as p29 in Paramecium, seems to increase ciliary beat frequency (CBF) directly. The mechanism by which cGMP increases CBF is less well defined but involves protein kinase G and possibly PKA. Protein kinase C inhibits ciliary beating. The regulation mechanisms of CBF by calcium remain somewhat controversial, favoring an immediate, direct action of calcium on ciliary beating and a second cyclic nucleotide-dependent phase. Finally, intracellular pH likely affects CBF through direct influences on dynein arms.

Whole body bone mineral content is similar at discharge from the hospital in premature infants receiving fortified breast milk or preterm formula

BACKGROUND

Prematurely born infants, especially those with very low birth weight (<1500 g) are at risk for metabolic bone disease.

OBJECTIVES

The influence of the type of oral feeding regimen and of other potential determinants of whole bone mineral content in prematurely born infants, when they approached full gestation, were evaluated. Previous studies have mainly examined effects at the level of regional bone.

METHODS

34 infants (21 males and 13 females), all born between 25.4 and 33.7 weeks of gestation, were studied before discharge. Whole body bone mineral content measurements were made just before hospital discharge using a commercial densitometer (Hologic QDR 4500, Hologic Inc, Waltham, MA) at a median age of 40 days (range, 10 to 115 days) after birth.

RESULTS

Expressed as a percentage of whole body mass, bone mass ranged between 0.86% and 1.99%, was similar between girls and boys and correlated positively with birth weight SD (r=0.42; P<0.05) and body weight SD (r=0.35; P<0.05). No difference in bone mass percentage was found between the different types of oral feedings (fortified human milk and preterm formula) or medications studied (corticoids and diuretics).

CONCLUSIONS

Whereas prenatal and postnatal weight gain determines the degree of bone mineralization of premature infants, it appears that the type of oral feeding does not affect differently the postnatal bone mineralization of premature infants, when assessed at the moment of discharge.

Bone and body composition measurements of small subjects: discrepancies from software for fan-beam dual energy X-ray absorptiometry

OBJECTIVES

A piglet model was used to determine the variations in measurements from different software algorithms used in the same type of dual energy X ray absorptiometry (DXA) instruments from the same manufacturer.

METHODS

Forty-one piglets (6190 +/- 5856g, mean +/- SD) were scanned in duplicate with a fan-beam densitometer (Hologic QDR4500A, Hologic Inc, Bedford, MA) in the infant whole body scan mode. The same scans were analyzed with two software versions: vKH6 (validated with carcass chemical measurement) and v11.2 (commercial software from the same densitometer manufacturer).

RESULTS

All analysis values were highly correlated (r = 0.90 to 1.00) and DXA values for total weights were almost identical. However, v11.2 results consistently overestimated bone mineral content (49.3 +/- 23.4%, mean +/- SD), bone area (21.1 +/- 8.2%), bone mineral density (24.1 +/- 22.2%), and fat mass (160.9 +/- 71.7%) but underestimated lean mass (-14.3 +/- 5.5%) when compared to the values from vKH6. Differences between software versions increased with heavier piglets.

CONCLUSION

The commercial software for fan-beam DXA measurement of piglets, matched for the size of human infants and young children, has major inaccuracies for bone mineral and body composition that become further exaggerated with increasing weight of the subject.

Resting energy expenditure in obese children aged 4 to 15 years: measured versus predicted data

AIM

To measure the relationship of resting energy expenditure (REE) and body composition, and to compare REE data calculated from anthropometric parameters using published equations with measurements obtained by indirect calorimetry (IC) in a population of obese paediatric patients.

METHODS

The study included 82 healthy obese paediatric subjects (49 boys, 33 girls; body mass index 29.6 +/- 5.0 kg/m , age 1 1.4 +/- 2.6 y, weight 72.4 +/- 20.9 kg, height 155 +/- 14 cm). REE was measured by IC, body composition was determined by dual energy X-ray absorptiometry (DXA). Bootstrap analysis was performed to validate the step-down linear regression analysis results.

RESULTS

Lean body mass (LBM) and weight were identified as the most significant determinants of REE. LBM was the best single predictor (r = 0.78; p < 0.001) for REE. Regression equations are given in the text. Prediction of REE on the basis of published anthropometric formulas was strongly dependent from the equation used. Some equations tend to underestimate REE in the population studied with a considerable systematic error.

CONCLUSION

In the present paper we show that (1) the published equations to predict REE in obese subjects yield scattered data and some are even biased by a systematic error, and that (2) the inclusion of DXA-derived LBM improves accuracy and precision of predicted REE in boys and girls aged from 4 to 10 y and in boys from 11 to 15 y.

Dual Energy X-Ray Absorptiometry Measurements in Small Subjects: Conditions Affecting Clinical Measurements

OBJECTIVE

To document the clinical and experimental situations that may affect DXA measurements in small subjects.

METHODS

49 piglets (886g to 21100g) had measurements with either of two pencil beam densitometers (QDR 1000W and QDR 2000 Plus, Hologic Inc, Waltham, MA) using commercial infant (IWB) and adult whole body (AWB) software v5.71p and v5.71 respectively. AWB scans were analyzed with three additional software versions. 35 infants (2115 to 11564g) had IWB measurements.

RESULTS

DXA measurements of total weight, bone mineral content, bone area, bone mineral density, fat and lean mass from IWB scans (all piglets) and from AWB scans (piglets >12 kg) were highly reproducible (p < 0.001). A statistically significant change occurred in at least one of the DXA measurements from the use of different platforms, variations in the amount and placement of covering (e.g., blanket), placement of the external calibration standard, presence of radiographic contrast material, presence of movement artifact, delivery of an intravenous fluid bolus prior to scanning or improper delineation of external calibration standard during analysis. Additionally, results varied amongst different versions of software as well as between IWB and AWB softwares.

CONCLUSION

In small subjects, consistency in the DXA techniques is paramount for valid and meaningful comparison of DXA data in bone mass and body composition.

Validation of a new pediatric air-displacement plethysmograph for assessing body composition in infants

BACKGROUND

The accurate measurement of body composition is useful in assessments of infant growth and nutritional status.

OBJECTIVE

This study evaluated the reliability and accuracy of a new air-displacement plethysmography (ADP) system for body-composition assessment in infants.

DESIGN

Between- and within-day reliability was assessed by comparing the percentage body fat (%BF) obtained on consecutive days and on the same day, respectively, in 36 full-term infants. Accuracy was assessed by comparing %BF measured with the use of ADP and %BF measured with the use of deuterium (2H2O) dilution in 53 infants.

RESULTS

There were no significant differences in %BF between days (-0.50 +/- 1.21%BF) or within days (0.16 +/- 1.44%BF). Mean between- and within-day test-retest SDs of 0.69 and 0.72%BF, respectively, indicated excellent reliability. The %BF measurements obtained by using ADP were not significantly influenced by infant behavioral state. Mean %BF obtained by using ADP (20.32%BF) did not differ significantly from that obtained by using 2H2O dilution (20.39%BF), and the regression line [%BF(2H2O) = 0.851%BF (ADP) + 3.094] gave a high R2 (0.76) and a low SEE (3.26). The 95% limits of agreement between ADP and 2H2O (-6.84%BF, 6.71%BF) were narrower than those reported for other body-composition techniques used in infants. Individual differences between the 2 methods were not a function of body mass or fatness.

CONCLUSION

ADP is a reliable and accurate instrument for determining %BF in infants, and it has the potential for use in both research and clinical settings.

Validation of Bone Mass and Body Composition Measurements in Small Subjects with Pencil Beam Dual Energy X-Ray Absorptiometry

OBJECTIVE

To validate the most widely reported dual energy X-ray absorptiometry (DXA) technique for the measurement of bone mass and body composition in human infants with a piglet model.

METHODS

Duplicate scans were obtained in 13 piglets (1950g to 21100g) using a whole body densitometer (Hologic QDR 2000 plus, Hologic Inc., Waltham, MA) operated in the pencil-beam mode on a two platform (aluminum platform overlying a foam table pad) system. DXA measurements that included total weight, bone mineral content, fat and lean mass were compared with carcass weight and chemical analysis for ash and calcium content, fat and lean mass.

RESULTS

Measurements from duplicate DXA scans were nearly perfectly correlated (r = 0.98 to 1.00). DXA measurements were strongly predictive of scale weight and chemical composition for all piglets (adjusted r(2) = 0.93 to 1.00, intraclass reliability coefficients = 0.943 to 0.999, p < 0.001 for all comparisons) although DXA bone mineral content consistently underestimated carcass ash and calcium content. Measured values from heavier piglets were not significantly different from values predicted from the lighter piglets' data. Slopes from regression based on lighter versus heavier piglets were not significantly different except for the bone mineral content with carcass ash or calcium content.

CONCLUSION

Our study validated the use of pencil beam DXA and its ability to determine relative changes in bone mass and body composition measurements over a much greater range of body weight than previous reports although its use as a direct indicator of nutrient requirement may be limited.

Evaluation of a new pediatric air-displacement plethysmograph for body-composition assessment by means of chemical analysis of bovine tissue phantoms

BACKGROUND

Body-composition assessment reflects infant growth and nutritional status but is limited by practical considerations, accuracy, and safety.

OBJECTIVE

This study evaluated the precision and accuracy of a new air-displacement plethysmography (ADP) system for pediatric body-composition assessment.

DESIGN

We used 24 phantoms constructed from bovine lean muscle and fat. The phantoms varied in mass (1.3894-9.9516 kg) and percentage fat (%Fat; 2.08-34.40%), thereby representing infants between birth and 6 mo of age. Estimates of %Fat obtained with chemical analysis (CA), hydrostatic weighing, and ADP were compared.

RESULTS

There was no significant difference between %Fat measured with ADP (%Fat(ADP)) and %Fat measured with CA (%Fat(CA)); the mean values were 18.55% and 18.59%, respectively. SDs for %Fat(ADP) and %Fat(CA) were not significantly different (0.70% and 0.73%, respectively). %Fat measurements obtained with ADP, CA, and hydrostatic weighing were highly correlated (r > 0.99, P < 0.0001). The regression equation (%Fat(CA) = 0.996%Fat(ADP) + 0.119; SEE = 0.600; adjusted R(2) = 0.997; P < 0.0001) did not differ significantly from the line of identity (%Fat(CA) = %Fat(ADP)). There was high agreement between individual measurements of %Fat(ADP) and %Fat(CA), as shown by the narrow 95% limits of agreements between methods (-1.22% to 1.13%), and there was no systematic bias in individual differences across the phantom mass and %Fat ranges.

CONCLUSION

ADP provides a highly precise and accurate estimate of %Fat in bovine tissue phantoms in the pediatric ranges of body weight and body fatness.

Body composition of the male and female reference infants

During infancy, especially early infancy, a substantial proportion of the requirements for energy and specific nutrients are those needed for growth. Knowledge of the body composition of a reference infant (body size and chemical composition at the 50th centile for age) permits an estimate of the growth needs of the infant. In this communication, we review efforts from the 1960s to the present at defining the composition of the male and female reference infants. We and others have demonstrated that accumulation of fat is remarkably rapid during the first 4 or 6 months of life. As a percentage of fat-free mass, water decreases throughout infancy whereas protein and minerals increase. However, the quantitative nature of these changes remains uncertain. After identifying the areas in which further data are needed, we conclude that the single most important area for further work is determining the relation of "bone mineral content" determined by dual energy X-ray absorptiometry to the osseous mineral content of the infant.

Nutritional assessment in preterm infants with special reference to body composition

In recent years, improvements in care have significantly improved survival in preterm and, particularily, the very low birth weight infant (VLBW). While immediate survival can be directly related to pulmonary maturity, several studies stress the importance of timely and adequate nutrition in these high-risk infants on a short- and long-term [1]. Yet, nutritional support remains a very controversial issue in these high-risk infants. Early provision of adequate intakes may be limited by clinical instability and immaturity. At the same time, nutritional requirements and methods of nutritional assessment are not well defined. The aim of this paper is to outline some of the methods used during nutritional assessment in preterm infants with special reference to the measurement of body composition.

Neonatal bone mass: influence of parental birthweight, maternal smoking, body composition, and activity during pregnancy

Evidence is accumulating that intrauterine growth and development may influence an individual's risk of osteoporosis in later adult life. To examine maternal and paternal influences on intrauterine skeletal growth, we used dual-energy X-ray absorptiometry to measure the neonatal bone mineral content (BMC) and bone mineral density (BMD) of 145 infants born at term. Independently of the infant's duration of gestation at birth, the birthweights of both parents and the height of the father were positively correlated with neonatal whole body BMC. Women who smoked during pregnancy had infants with a lower whole body BMC and BMD; overall, there was a 7.1-g (11%) average difference between whole body BMC of infants whose mothers did and did not smoke during pregnancy (p = 0.005). Women with thinner triceps skinfold thicknesses (reflecting lower fat stores) and those who reported a faster walking pace and more frequent vigorous activity in late pregnancy also tended to have infants with a lower BMC and BMD (p values for BMC; 0.02, 0.03, and 0.05, respectively). Maternal thinness and faster walking pace but not maternal smoking or parental birthweight also were associated with lower bone mineral apparent density (BMAD). The influences on skeletal growth and mineralization were independent of placental weight, a marker of the placental capacity to deliver nutrients to the fetus. These observations point to a combination of genetic and intrauterine environmental influences on prenatal skeletal development and suggest that environmental modulation, even at this early stage of life, may reduce the risk of osteoporosis in adulthood.

Weight gain composition in preterm infants with dual energy X-ray absorptiometry

Whole body composition was investigated using dual energy x-ray absorptiometry in 54 healthy preterm infants, birth weight < 1750 g, who were fed fortified human milk (n = 20) and preterm formula (n = 34) when full enteral feeding was attained and then again 3 wk later at around the time of discharge. Weight gain composition was calculated from the difference between the earlier and later measurement. The minimal detectable changes in whole body composition over time according to the variance of the population (within groups of 20 infants) and the minimal detectable changes according to the dietary intervention (between two groups of 20 infants) were determined at 5% significance and 80% power. Whole body composition was similar in the two groups at the initial measurement, but all the measured variables differed at the time of the second measurement. Formula-fed infants showed a greater weight gain (19.9 +/- 3.2 versus 15.9 +/- 2.2 g.kg(-1).d(-1), p < 0.05), fat mass deposition (5.1 +/- 1.9 versus 3.3 +/- 1.3 g.kg(-1).d(-1), p < 0.05), bone mineral content gain (289 +/- 99 versus 214 +/- 64 mg.kg(-1).d(-1), p < 0.05), and increase in bone area (1.6 +/- 0.4 versus 1.3 +/- 0.3 cm(2).kg(-1).d(-1), p < 0.05) compared with the fortified human milk group. From these data, a minimal increase from the first measurement of 111 g lean body mass, 68 g fat mass, and 3. 1 g bone mineral content is needed to be detectable in a longitudinal study that includes 20 infants. For significance between two groups of 20 infants around the time of discharge, dietary intervention needs to achieve minimal differences of 160 g lean body mass, 86 g fat mass, and 4.1 g bone mineral content. With respect to weight gain composition, the minimal differences required to reach significance are 2.1 g.kg(-1).d(-1) for gain in lean body mass, 1.2 g.kg(-1).d(-1) for gain in fat mass, and 76 mg.kg(-1). d(-1) for gain in bone mineral content. We conclude that dual energy x-ray absorptiometry allows evaluation of the effects of dietary intervention on whole body and weight gain composition in preterm infants during the first weeks of life.

Body composition in human infants at birth and postnatally

The predictive values of anthropometric measurements, race, gender, gestational and postnatal ages, and season at birth and at study for the total body dual energy X-ray absorptiometry (DXA)-derived lean mass (LM), fat mass (FM) and fat mass as a percentage of body weight (%FM) were determined in 214 singleton appropriate birth weight for gestational age infants [101 Caucasian (60 boys, 41 girls) and 113 African American (55 boys, 58 girls)]. Gestational ages were 27-42 wk and the infants were studied between birth and 391 d, weighing between 851 and 13446 g. In addition, predictive value of body weight, LM and FM for DXA bone measurements was also determined. Scan acquisition used Hologic QDR 1000/W densitometer and infant platform and scans without significant movement artifacts were analyzed using software 5.64p. Body weight, length, gender and postnatal age were significant predictors of LM (adjusted R:(2) >0. 94) and FM (adjusted R:(2) >0.85). Physiologic variables had little predictive value for %FM except in the newborns (adjusted R:(2) 0. 69). Body weight was the dominant predictor of LM and FM, although length had similar predictive value for LM with increasing postnatal age. Female infants had less LM and more FM throughout infancy (P: < 0.01). LM or FM offered no advantage over body weight in the prediction of bone mass measurements. DXA is a useful means with which to determine body composition, and our data are important in the design and assessment of nutritional intervention studies.

Body composition measurements during infancy

Infancy is the period of most rapid postnatal growth and is accompanied by major changes in body composition (BC). There are many challenges to successfully measuring BC of infants in vivo, which include the inherent limitations in the underlying assumptions for each technique. The small body mass and rapid nonuniform changes in body parts, that is, the components of BC during infancy, can strain the technical limits of all methods. Many techniques for in vivo BC measurement used in older people have been applied to infants. However, the vast majority of them either are difficult to adapt for widespread use in infants, or the roles and limitations for using them during infancy are ill-defined because of limited or no critical validation and cross-calibration studies. Based on validation data from animals, well-defined methodological issues in data acquisition and analyses, availability of normative data, and pertinent accuracy and precision of the technique to allow us to determinate clinically relevant changes in BC within a reasonable time interval, three techniques appear to be most suitable for in vivo BC measurement in infants. Anthropometric measurements can be used in field studies or for group comparisons, and total body electrical conductivity (TOBEC) and selected dual-energy X-ray absorptiometry (DXA) measurements can be used to compare BC in individual infants. DXA has the advantages of being able to measure bone mass and the potential to be adaptable to the widely available existing instruments. However, regardless of the techniques used in measuring BC in infants, meticulous attention to details in data acquisition and data analysis, and a knowledge of the limitations of the particular technique are the prerequisites for generating valid data.