Computing for Research ISpring 2012

Exploratory Data Analysis and Hypothesis TestingFebruary 21

Primary Instructor:Elizabeth Garrett-MAyer

Exploratory Data Analysis

• We’ve already discussed some basic stuff– sum and sum, detail– tab

• What other sorts of exploration might we do?• Confidence intervals– for continuous variables– for categorical variables

Immediate command for CIs

Continuous:cii N xbar s

Binary:cii N phatorcii N x

Confidence intervals

• For a continuous variable:mean varlist

• Example:* estimate means of ceramide variablesmean c18ceramidemean totalc - s1pc1

Additional options

tab initialre initialmean c18ceramide, over(initialre)

mean c18ceramide, vce(bootstr)

mean c18ceramide, vce(bootstr) over(initialre)mean c18ceramide, over(initialre)

mean c18ceramide, level(90)

Confidence intervals for proportion

proportion varlist

Examplesproportion failure

proportion failure death initialre

proportion failure, vce(bootstr)proportion failure, cluster(patient)proportion failure, level(90)

Hypothesis Testing

• A number of different approaches• Options– nonparametric vs. parametric– continuous vs. categorical (vs. other?)– one vs. two vs. more than two groups

One sample t-tests

• ttesti N mean sd null • ttest varname == null• ttest var1 == var2 *paired

• Examples:ttesti 20 48 2.75 50 ttest c18c == 10ttest frombaselines1p==100ttest frombaselinec18==100

Two sample t-teststtesti N1 mean1 sd1 N2 mean2 sd2ttest varname1 == varname2, unpairedttest varname, by(groupvar)

Examples:

ttest c18, by(sex)ttest c18, by(sex) unequal

Nonparametric?

• ranksum: two group comparison• kwallis: >= two group comparison

• signrank: matched pairs signed ranks test• signtest: sign test of matched pairs

Nonparametric?*nonparametric testsranksum c18, by(sex)kwallis c18, by(sex)

use ceramide.alldata, clearkeep if cycle==3gen c18dif = frombaselinec18-100signrank c18dif=0signrank frombaselinec18=100signtest c18dif=0signtest frombaselinec18=100

Anova

• anova y x(note that x is assumed to be categorical)anova y x1 x2

Examples:anova c18c initialre

One sample binomial tests

• prtest and bitest• Difference? – prtest uses large sample approximations– bitest uses exact test

bitest varname==p0bitesti N x p0

One sample binomial testsuse "SCBC2004.v9.dta", clearreplace ercat=. if ercat==9gen ercatn=cond(ercat==2,0,1)replace ercatn=. if ercat==.tab ercat ercatn

bitest ercatn=0.50bitest ercatn=0.65prtest ercatn=0.65

Two (or more) sample binomial tests

tab y x, exacttab y x, chi

tab ercatn gradetab ercatn stage