Descriptive Statistics for Continuous Variables

Summary of a Continuous Variable

summary(varname)
Provides minimum, 1st quartile (25%tile), median, mean, 3rd quartile (75%tile) and maximum values of varname.
summary(age)
**Min. 1st Qu. Median *Mean 3rd Qu. *Max.
61.00 **63.00 *66.00 *66.94 **70.00 75.00

Mean

mean(varname)
mean(age)
[1] 66.94286

Standard Deviation

sd(varname)
sd(age)
[1] 4.458586

Median

median(varname)
median(age)
[1] 66

 Descriptive Statistics by Subsets

To compute descriptive statistics for continuous variables by group, use the tapply() function

tapply(varname, groupname, statistic)

Provides statistics for varname by groupname.   Statistics can be summary, mean, sd and others.

tapply(bmi,anyS,mean)
This gives the mean of bmi separately for those with variable anyS=0 and anyS=1.

    ***0       1
23.4358 23.6796

Similarly, we can compute the standard deviation for bmi in each of these two groups.

tapply(bmi,anyS,sd)

      **0        1
4.971052 4.665697