t-tests

The t.test( ) function produces a variety of t-tests. Unlike most statistical packages, the default assumes unequal variance and applies the Welsh df modification.# independent 2-group t-test
t.test(y~x) # where y is numeric and x is a binary factor

# independent 2-group t-test
t.test(y1,y2) # where y1 and y2 are numeric

# paired t-test
t.test(y1,y2,paired=TRUE) # where y1 & y2 are numeric

# one sample t-test
t.test(y,mu=3) # Ho: mu=3

You can use the var.equal = TRUE option to specify equal variances and a pooled variance estimate. You can use the alternative="less" or alternative="greater" option to specify a one tailed test.

Nonparametric and resampling alternatives to t-tests are available.

Visualizing Results

Use box plots or density plots to visualize group differences.