Measures of position and variability

To produce the mean, median, minimum, maximum values, 1st and 3rd quartiles for maximum length, use

> summary(Maxle)

To calculate the standard deviation for a variable:

> sd(Maxle, na.rm=TRUE)

The na.rm=TRUE parameter is necessary because we have missing values in our measurements.

The boxplot is based on some of the values given by the summary function. Its central line represents the average mean. To produce a boxplot for maximum length:

> boxplot(Maxle)

As above but for each material:

> tapply(Maxle,Mat,summary)
> boxplot(split(Maxle,Mat))
> tapply(Maxle,Mat,sd,na.rm=TRUE)