short- and long - term variability in outcomes can affect power in research studies . weight is usually measured on a single day per study visit ( i.e. , time point in study : baseline and followup ) in clinical trials of weight loss or maintenance .   however , day - to - day variability in weight within an individual may decrease precision , thus impacting necessary sample sizes . because of this type of variability in blood pressure , sample size is optimized by using at least 3 sets of daily blood pressure measurements per study visit . we aimed to evaluate the hypothesis that measurement of weight on one day is sufficiently precise compared to two days by ( 1 ) comparing the standard deviation of weight change based on one versus two days of measurements and ( 2 ) evaluating components of weight variability using data from the weight loss maintenance ( wlm ) trial . wlm was an nhlbi - supported , 4-site , and randomized controlled trial comparing 2 weight loss maintenance interventions to a self - directed control group in two phases :   an initial 6-month weight - loss phase common to all participants ( phase 1 ) and a randomized , 30-month weight - maintenance phase ( phase 2 ) . participants had a body mass index ( bmi ) between 25 and 45  kg / m , treated hypertension , hyperlipidemia , or both , access to a telephone and the internet , no active cardiovascular disease , diabetes , or medical condition(s ) precluding participation , absence of > 9  kg weight loss in the prior 3 months , no use of weight loss medication , and no history of weight loss surgery . participants provided informed consent , and institutional review boards at each study site approved the protocol . participants were eligible for phase 2 if they lost 4  kg during the initial weight loss phase . the current study is limited to the 316 ( out of 342 ) control group participants who attended either the 12- or 30-month follow - up visits in phase 2 . this group received printed lifestyle guidelines ( for diet and physical activity ) while meeting with an interventionist at randomization   and after the 12-month visit . weight was measured during screening , at the conclusion of phase 1 ( baseline ) , and at 12- and 30-month postrandomization into phase 2 . staff used high - quality calibrated digital scales and calibrated stadiometers , and participants wore light clothes and no shoes .    on any given day , weight was measured as the average of two independent measurements .   for the baseline and for the 12- and 30-month visits , this process was repeated on two separate days ( day 1 and day 2 ) .   at baseline , these repeated measurements could have been separated by as much as 32 days , and for purposes of this analysis ,   we restricted   participants to those individuals whose baseline weight measures were within 21 days apart from each other . only 5 ( 1.6% ) out of a possible 314 participants were excluded from the 12-month analysis for this reason , and only 4 ( 1.4% ) out of a possible 285 participants were excluded from the 30-month analysis .   because the baseline weights were taken during the period of active weight loss intervention when the cohort as a whole was still losing weight , we adjusted the initial ( day 1 ) weights to have the same mean as the final weights ( day 2 ) by adding to each person 's day-1 weight the mean change in weight across all participants between day 1 and 2 .   for the 12- and 30-month visits , adjustment for temporal drift between days 1 and 2 was not needed since participants were not undergoing an active weight loss intervention . the mean differences in the two weights at 12- and 30-months were 0.02 and 0.03  kg , respectively . participants were not required to fast , and weight measurement did not occur at a specific time of day . however , each participant was required to present in a fasting state for at least one day per visit which included laboratory testing ; this visit would likely occur in the morning . analyses were completed using sas 9.2 .   using ( a ) the first and ( b ) both daily weights from each visit , we calculated the mean and variance in weight change from baseline to 12 months and from baseline to 30 months . we defined the variance in weight change as the standard deviation of weight change and used f - tests to compare them . we conducted similar analyses stratified by screening bmi ( 2529.9  kg / m and 30  kg / m ) , sex , and race ( african american and non - african american ) . we also conducted a nested analysis of variance to estimate between - person , between - visit , and day - to - day ( within - visit ) variability in weight .   within - day mean ( sd ) age was 55.8 ( 8.5 ) years , and mean ( sd ) bmi was 30.8 ( 4.5 )  kg / m ; 64% of participants were women ; 36% were african american ; 50% were obese ( table 1 ) . the mean number of days between day 1 and day 2 of weight measurement for at the baseline , 12-month , and 30-month visits was 8.9 , 9.1 , and 10.4 , respectively . although the standard deviation of weight change differed for measurements taken 12 and 30 months apart , it was essentially unchanged regardless of whether weight was measured as the mean of one or two days of measurements ( table 1 ) . for example , the standard deviation for 30-month weight change for all participants was 6.3  kg when weight was measured on a single day ( day 1 ) and 6.3  kg when weight was measured on two separate days ( days 1 and 2 , p = .98 ) . we found no statistically significant differences overall or in any of the subgroups studied . day - to - day variability ( i.e. , of mean change in weight from a single day compared to that using both day 1 and 2 of the visits ) within individuals   accounted for < 1% of total variability in weight , while between - visit variability within individuals accounted for 617% of the variability in weight change depending on the subgroup ( table 1 ) . in the context of this study being conducted with a rigorous weight measurement protocol , the use of one versus two measurement days per study visit ( time point ) had essentially no impact on the variability of weight change in adults measured over 12 and 30 months , and day - to - day variability in weight correspondingly accounted for less than 1% of the total variability in weight . a limitation of the study is that the baseline weights were measured while the participants were still engaged in an active weight - loss intervention . we minimized the added variability in weight caused by this by adjusting for temporal drift and by only including participants with 21 or fewer days between day 1 and 2 measurements at the baseline visit . also , we evaluated the contribution of day - to - day variability in weight to variability in weight change over 12 and 30 months . day - to - day variability in weight may be of more significance in shorter studies thus limiting the generalizability of our results to studies brief in duration . to our knowledge , this is the first study of its kind to evaluate day - to - day variability in weight as it applies to measurement in clinical studies .   especially pertinent is our subgroup analysis showing the stability of day - to - day weight variability across subgroups defined by bmi , sex , and race . finally , wlm used a stringent weight measurement protocol that included calibrated digital scales , trained staff , and duplicate within - day measurements to minimize the probability of measurement or transcription error .   observed day - to - day variability could be expected to be greater in studies with less stringent weight measurement protocols , though our findings would suggest such increased variability would likely be due to measurement error rather than to true day - to - day variability . in conclusion , we show that in the setting of a well - standardized weight measurement protocol with negligible measurement error , there is little day - to - day variability in weight . weight measurement on a single day is sufficiently precise for use in a clinical trial .
background .   weight is typically measured on a single day in research studies . 
 this practice assumes negligible day - to - day weight variability , although little evidence exists to support this assumption .   we compared the precision of measuring weight on one versus two days among control participants in the weight loss maintenance trial . 
 methods . trained staff measured weight on two separate days at baseline , 12 months , and 30 months ( 20042007 ) . 
 we calculated the standard deviation ( sd ) of mean weight change from baseline to the 12- and 30-month visits using ( a ) the first and ( b ) both daily weights from each visit and conducted a variance components analysis ( 2009 ) . 
 results . of the 316 participants with follow - up measurements , 
 mean ( sd ) age was 55.8 ( 8.5 ) years , bmi was 30.8 ( 4.5 )  kg / m2 , 64% were women , 36% were black , and 50% were obese .   at 12 months , 
 the sd of mean weight change was 5.1  versus 5.0  kg using one versus two days of weight measurements ( p = .76 ) , while at 30 months the corresponding sds were 6.3 and 6.3  kg ( p = .98 ) . 
 we observed similar findings within subgroups of bmi , sex , and race . 
 day - to - day variability within individuals accounted for < 1% of variability in weight . 
 conclusions . 
 measurement of weight on two separate days has no advantage over measurement on a single day in studies with well - standardized weight measurement protocols .