Compute various measures of internal consistencies for tests or item-scales of questionnaires.

## Arguments

- x
Depending on the function,

`x`

may be a`matrix`

as returned by the`cor()`

-function, or a data frame with items (e.g. from a test or questionnaire).

## Value

A data frame with three columns: The name(s) of the item(s), the item difficulties for each item, and the ideal item difficulty.

## Details

This function calculates the item difficulty, which should
range between 0.2 and 0.8. Lower values are a signal for
more difficult items, while higher values close to one
are a sign for easier items. The ideal value for item difficulty
is `p + (1 - p) / 2`

, where `p = 1 / max(x)`

. In most
cases, the ideal item difficulty lies between 0.5 and 0.8.