# testRetestCES

##### testRetestCES

The testRetestCES function computes the test-retest Coefficient of Equivalence and Stability (Schmidt, Le & Ilies, 2003).

##### Usage

```
testRetestCES(dat = NULL, moments = NULL, testDat = NULL, retestDat = NULL, parallelTests = 'means', sortItems = FALSE, convertToNumeric = TRUE, digits=4)
parallelSubscales(dat, convertToNumeric = TRUE)
```

##### Arguments

- dat
- A dataframe. For testRetestCES, this dataframe must contain 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. The number of columns in this dataframe MUST be even! Note that instead of providing this dataframe, the items of each measurement moment can be provided separately in testDat and retestDat as well.
- 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).
- parallelTests
- A vector indicating which items belong to which parallel test; like the moments vector, this should have two possible values (e.g. 1 and 2). Alternatively, it can be character value with 'means' or 'variances'; in this case, parallelSubscales will be used to create roughly parallel halves.
- 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.
- digits
- Number of digits to print.

##### Details

This function computes the test-retest Coefficient of Equivalence and Stability (CES) as described in
Schmidt, Le & Ilies (2003). Note that this function only computes the test-retest CES for a scale that
is administered twice and split into two parallel halves post-hoc (this procedure is explained on page
210, and the equations that are used, 16 and 17a are explained on page 212).

##### Value

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

##### Note

This function uses equations 16 and 17 on page 212 of Schmidt, Le & Ilies (2003): in other words, this function assumes that one scale is administered twice. If you'd like the computation for two different but parellel scales/measures to be implemented, please contact me.

##### References

Schmidt, F. L., Le, H., & Ilies, R. (2003) Beyong Alpha: An Empirical Examination of the Effects of Different Sources of Measurement Error on Reliability Estimates for Measures of Individual-differences Constructs. Psychological Methods, 8(2), 206-224. doi:10.1037/1082-989X.8.x.206

##### Examples

```
## Not run:
# ### This will prompt the user to select an SPSS file
# testRetestCES();
# ## 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
testRetestCES(exampleData);
```

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