tTestLnormAltRatioOfMeans {EnvStats}  R Documentation 
Compute the minimal or maximal detectable ratio of means associated with a one or twosample ttest, given the sample size, coefficient of variation, significance level, and power, assuming lognormal data.
tTestLnormAltRatioOfMeans(n.or.n1, n2 = n.or.n1, cv = 1, alpha = 0.05, power = 0.95,
sample.type = ifelse(!missing(n2), "two.sample", "one.sample"),
alternative = "two.sided", two.sided.direction = "greater", approx = FALSE,
tol = 1e07, maxiter = 1000)
n.or.n1 
numeric vector of sample sizes. When 
n2 
numeric vector of sample sizes for group 2. The default value is the value of

cv 
numeric vector of positive value(s) specifying the coefficient of
variation. When 
alpha 
numeric vector of numbers between 0 and 1 indicating the Type I error level
associated with the hypothesis test. The default value is 
power 
numeric vector of numbers between 0 and 1 indicating the power
associated with the hypothesis test. The default value is 
sample.type 
character string indicating whether to compute power based on a onesample or
twosample hypothesis test. When 
alternative 
character string indicating the kind of alternative hypothesis. The possible values
are 
two.sided.direction 
character string indicating the direction (greater than 1 or less than 1) for the
detectable ratio of means when 
approx 
logical scalar indicating whether to compute the power based on an approximation to
the noncentral tdistribution. The default value is 
tol 
numeric scalar indicating the toloerance to use in the

maxiter 
positive integer indicating the maximum number of iterations
argument to pass to the 
If the arguments n.or.n1
, n2
, cv
, alpha
, and
power
are not all the same length, they are replicated to be the same length
as the length of the longest argument.
Formulas for the power of the ttest for lognormal data for specified values of
the sample size, ratio of means, and Type I error level are given in
the help file for tTestLnormAltPower
. The function
tTestLnormAltRatioOfMeans
uses the uniroot
search algorithm
to determine the required ratio of means for specified values of the power,
sample size, and Type I error level.
a numeric vector of computed minimal or maximal detectable ratios of means. When
alternative="less"
, or alternative="two.sided"
and
two.sided.direction="less"
, the computed ratios are less than 1
(but greater than 0). Otherwise, the ratios are greater than 1.
See tTestLnormAltPower
.
Steven P. Millard (EnvStats@ProbStatInfo.com)
See tTestLnormAltPower
.
tTestLnormAltPower
, tTestLnormAltN
,
plotTTestLnormAltDesign
, LognormalAlt,
t.test
, Hypothesis Tests.
# Look at how the minimal detectable ratio of means for the onesample ttest
# increases with increasing required power:
seq(0.5, 0.9, by = 0.1)
#[1] 0.5 0.6 0.7 0.8 0.9
ratio.of.means < tTestLnormAltRatioOfMeans(n.or.n1 = 20,
power = seq(0.5, 0.9, by = 0.1))
round(ratio.of.means, 2)
#[1] 1.47 1.54 1.63 1.73 1.89
#
# Repeat the last example, but compute the minimal detectable ratio of means
# based on the approximate power instead of the exact:
ratio.of.means < tTestLnormAltRatioOfMeans(n.or.n1 = 20,
power = seq(0.5, 0.9, by = 0.1), approx = TRUE)
round(ratio.of.means, 2)
#[1] 1.48 1.55 1.63 1.73 1.89
#==========
# Look at how the minimal detectable ratio of means for the twosample ttest
# decreases with increasing sample size:
seq(10, 50, by = 10)
#[1] 10 20 30 40 50
ratio.of.means < tTestLnormAltRatioOfMeans(seq(10, 50, by = 10), sample.type="two")
round(ratio.of.means, 2)
#[1] 4.14 2.65 2.20 1.97 1.83
#
# Look at how the minimal detectable ratio of means for the twosample ttest
# decreases with increasing values of Type I error:
ratio.of.means < tTestLnormAltRatioOfMeans(n.or.n1 = 20,
alpha = c(0.001, 0.01, 0.05, 0.1), sample.type = "two")
round(ratio.of.means, 2)
#[1] 4.06 3.20 2.65 2.42
#==========
# The guidance document Soil Screening Guidance: Technical Background Document
# (USEPA, 1996c, Part 4) discusses sampling design and sample size calculations
# for studies to determine whether the soil at a potentially contaminated site
# needs to be investigated for possible remedial action. Let 'theta' denote the
# average concentration of the chemical of concern. The guidance document
# establishes the following goals for the decision rule (USEPA, 1996c, p.87):
#
# Pr[Decide Don't Investigate  theta > 2 * SSL] = 0.05
#
# Pr[Decide to Investigate  theta <= (SSL/2)] = 0.2
#
# where SSL denotes the preestablished soil screening level.
#
# These goals translate into a Type I error of 0.2 for the null hypothesis
#
# H0: [theta / (SSL/2)] <= 1
#
# and a power of 95% for the specific alternative hypothesis
#
# Ha: [theta / (SSL/2)] = 4
#
# Assuming a lognormal distribution, the above values for Type I and power, and a
# coefficient of variation of 2, determine the minimal detectable increase above
# the soil screening level associated with various sample sizes for the onesample
# test. Based on these calculations, you need to take at least 6 soil samples to
# satisfy the requirements for the Type I and Type II errors when the coefficient
# of variation is 2.
N < 2:8
ratio.of.means < tTestLnormAltRatioOfMeans(n.or.n1 = N, cv = 2, alpha = 0.2,
alternative = "greater")
names(ratio.of.means) < paste("N=", N, sep = "")
round(ratio.of.means, 1)
# N=2 N=3 N=4 N=5 N=6 N=7 N=8
#19.9 7.7 5.4 4.4 3.8 3.4 3.1
#
# Repeat the last example, but use the approximate power calculation instead of
# the exact. Using the approximate power calculation, you need 7 soil samples
# when the coefficient of variation is 2. Note how poorly the approximation
# works in this case for small sample sizes!
ratio.of.means < tTestLnormAltRatioOfMeans(n.or.n1 = N, cv = 2, alpha = 0.2,
alternative = "greater", approx = TRUE)
names(ratio.of.means) < paste("N=", N, sep = "")
round(ratio.of.means, 1)
# N=2 N=3 N=4 N=5 N=6 N=7 N=8
#990.8 18.5 8.3 5.7 4.6 3.9 3.5
#==========
# Clean up
#
rm(ratio.of.means, N)