The case-control matching procedure is used to randomly match cases and controls based on specific criteria. MedCalc can match on up to 4 different variables.
In the example we will use the following data:
The treated cases are coded 1, the controls are coded 0. For each treated case MedCalc will try to find a control case with matching age and gender.
- Classification variable: select or enter a dichotomous variable indicating group membership (0=control, 1=case).If your data are coded differently, you can use the Define status tool to recode your data.
- Variable with case identification: select a variable that contains a unique identification code for each subject in the spreadsheet. If you do not select a variable here (not recommended), MedCalc will use row numbers as case identification.
- Match on: select up to 4 variables and for each variable the maximum allowable difference (caliper). Smaller calipers will result in reduced bias and closer matches, but may also result in a smaller number of matches. Select the option "Exact match" to match on a variable that is not numerical (for example, a variable "Gender" that is coded 'Male' and 'Female').
- Filter: (optionally) a filter in order to include only a selected subgroup of cases (e.g. SEX="Male").
- Advanced: click the Advanced button to enter the number of iterations and the random number seed.
For the example data, we complete the dialog box as follows:
The results are displayed in a dialog box.
The program gives the total number of subjects, number of cases, number of controls and the number of matched cases, i.e. the number of cases for which a matching control has been found.
Next, the mean difference between the matched subjects are given, with mean difference, SD, 95% CI of the difference and associated P-value (paired samples t-test). The 95% confidence intervals should be small and neglectable. P-values should be non-significant. If for one or more variables the confidence interval is large or the P-value is significant, the "maximum allowable difference" entered in the input dialog box (see above) was probably too large.
Save match IDs in spreadsheet column
Click the "Save match IDs..." button to create a new column in the spreadsheet with for each case the identification of the matched control (and vice-versa).
A column is added in the spreadsheet:
In subsequent statistical analyses this new column can be used in a filter in order to include only cases and controls for which a match was found.
E.g. if the new column has MatchID as a heading, the filter could be MatchID>0 or MatchID<>"" (<> means Not Equal To).
Save as new file with paired data
Click the "Save new file..." button to create a new MedCalc data file in which the data are rearranged as follows:
- The file includes the data of cases with matching controls only.
- A first set of columns contains the data of the cases. The heading of these columns is the original heading with "_T" appended. A second set of columns contains the data of the controls.The heading of these columns is the original heading with "_C" appended.
- On each row, the data of a case and its matching control is given.
This new datafile will allow to perform statistical tests on paired data.