# testRetestAlpha

##### testRetestAlpha

The testRetestAlpha function computes the test-retest alpha coefficient (Green, 2003).

##### Usage

`testRetestAlpha(dat = NULL, moments = NULL, testDat = NULL, retestDat = NULL, sortItems = FALSE, convertToNumeric = TRUE)`

##### Arguments

- dat
- A dataframe containing the items in the scale at both measurement moments. If no dataframe is specified, a dialogue will be launched to allow the user to select an SPSS datafile. If only one dataframe is specified, either the items have to be ordered chronologically (i.e. first all items for the first measurement, then all items for the second measurement), or the vector 'moments' has to be used to indicate, for each item, to which measurement moment it belongs.
- moments
- Used to indicate to which measurement moment each item in 'dat' belongs; should be a vector with the same length as dat has columns, and with two possible values (e.g. 1 and 2).
- testDat, retestDat
- Dataframes with the items for each measurement moment: note that the items have to be in the same order (unless sortItems is TRUE).
- sortItems
- If true, the columns (items) in each dataframe are ordered alphabetically before starting. This can be convenient to ensure that the order of the items at each measurement moment is the same.
- convertToNumeric
- When TRUE, the function will attempt to convert all vectors in the dataframes to numeric.

##### Details

This function computes the test-retest alpha coefficient as described in Green (2003).

##### Value

- An object with the input and several output variables. Most notably:

##### References

Green, S. N. (2003). A Coefficient Alpha for Test-Retest Data. Psychological Methods, 8(1), 88-101. doi:10.1037/1082-989X.8.1.88

##### Examples

```
## Not run:
# ### This will prompt the user to select an SPSS file
# testRetestAlpha();
# ## End(Not run)
### Load data from simulated dataset testRetestSimData (which
### satisfies essential tau-equivalence).
data(testRetestSimData);
### The first column is the true score, so it's excluded in this example.
exampleData <- testRetestSimData[, 2:ncol(testRetestSimData)];
### Compute test-retest alpha coefficient
testRetestAlpha(exampleData);
```

*Documentation reproduced from package userfriendlyscience, version 0.5-2, License: GPL (>= 2)*

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