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Explanations and References
This page describes the relationship between sample size and error in estimating Standard Deviations (SD) from a sample
Sample Size Table
As most statistical calculations are based on the critical assumption that the SD of a measurement is known and valid, the precision of an estimated SD is therefore important. In many cases in the clinical situation, an assumed value for SD is used, based on published figures or from estimates using a small pilot study, but this often results in misleading conclusions In particular, in engineering and in high precision biochemical laboratories, a valid assumed SD is critical, and the calculations presented in this panel is one of the methods used to obtain this. The error, the confidence interval of the SD estimated, is expressed as a percent of the value. The greater the sample size, the narrower would be this confidence interval. Two calculations are offered in this panel
ReferencesGreenwood JA and Sandomire MM (1950) Journal of the American Statistical Association 45 (250) p. 257 - 260 Burnett RW (1975)Accurate estimation of standard deviations for quantitative methods used in clinical chemistry. Clin. Chem. 21 (13) p. 1935-1938
This panel presents 2 tables related to sample size and Standard Deviations. The first estimates the sample size required at the planning stage, and the second the confidence interval from the data obtained
Javascript Program
Table of Sample size
This sub-panel provides the R codes for estimating sample size needed and error estimated with Standard Deviation, algorithm is as described in
Section 1. Supportive subroutines ChiSqToP<-function(chi,degF) #function to calculate probability from chisq and df { return (1 - pchisq(chi, df=degF)) #probability } Confidence <- function (u,df) # common to both ssiz and error { #x1 = pow(1 - u,2) * df #x2 = pow(1 + u,2) * df x1 = (1 - u)^2 * df x2 = (1 + u)^2 * df return (ChiSqToP(x1,df) - ChiSqToP(x2,df)); } # Calculate sample size from confidence and % error SSizSD <- function (Cf,Er) # percent confidence, tolerable error (as % of SD found) { Cf = Cf / 100; # converts to probability Er = Er / 100; # converts to probability # iterate for correct sample size nl = 0; nr = 100000000; nm = nr / 2; p = Confidence(Er,round(nm)); while(abs(nl-nr)>1) { if(p<Cf) { nl = nm; } else { nr = nm; } nm = (nr + nl) / 2.0; p = Confidence(Er,round(nm)); } #print(ceiling(nm) + 1) return (ceiling(nm) + 1); # nm is df so sample size needs plus 1 } # Calculate Error from sample size (n) and % confidence ErrSD <- function(cf,ssiz) # n= sample size, Cf = percent confidence { cf = cf / 100; # converts to probability df = ssiz-1; # degrees of confidence # iterate for error ul = 0; ur = 1; um = 0.5; p = Confidence(um,df); while(abs(ul-ur)>0.0001) { if(p>cf) { ur = um; } else { ul = um; } um = (ul + ur) / 2.0; p = Confidence(um,df); } return (um * 100); }Section 2. Main Programs Propgram 1. Sample size txt = (" Cf Err 90 10 95 5 99 1 ") # input data Cf=% confidence Err= error in % of SD df <- read.table(textConnection(txt),header=TRUE) #df # optional display of input data frame # extract arrays from data frame arCf <- df$Cf arErr <- df$Err # Create results array arSSiz <- vector() # Calculate for(i in 1:nrow(df)) { cf = arCf[i] er = arErr[i] arSSiz <- append(arSSiz,SSizSD(cf, er)) # append sample size to result array } # Incorporatw results to original data frame df$SSiz <- arSSiz # show data frame with results dfThe results are as follows > df Cf Err SSiz 1 90 10 137 2 95 5 769 3 99 1 33175Propgram 2. Error as % of SD found txt = (" Cf SSiz 90 137 95 769 99 33175 ") # input data Cf=% confidence, SSiz = sample size df <- read.table(textConnection(txt),header=TRUE) #df # optional display of input data frame # extract arrays from data frame arCf <- df$Cf arSSiz <- df$SSiz # Create results array arEr <- vector() # Calculate for(i in 1:nrow(df)) { cf = arCf[i] ssiz = arSSiz[i] print(c(cf,ssiz)) arEr <- append(arEr,ErrSD(cf, ssiz)) # append error to result array } # Incorporatw results to original data frame df$Error <- arEr # show data frame with results dfThe results are > df Cf SSiz Error 1 90 137 9.9700928 2 95 769 5.0018311 3 99 33175 0.9979248 |