Computing dprimes and betas
Dprimes and Betas may be computed in SPSS using the probit function. The UCLA statistics department give further details. They suggest values upto 2 are typical dprime values.
A table showing dprime values for true positives and false alarm combinations is available.
- Also available by typing sigdet at a UNIX prompt
Excel syntax available here.
[CUT AND PASTE SPSS SYNTAX BELOW INTO A SYNTAX WINDOW AMENDING DATA AS REQUIRED]
* Computes Dprimes and Betas for signal detection theory * amend data input as required SET FORMAT F10.5. DATA LIST FREE / HI FN FP CN. BEGIN DATA 9 1 3 7 6 4 2 8 END DATA. IF (HI EQ 0) HI = 0.5. IF (FN EQ 0) FN = 0.5. IF (FP EQ 0) FP = 0.5. IF (CN EQ 0) CN = 0.5. COMPUTE HIP = HI / (HI + FN). COMPUTE FPP = FP / (FP + CN). COMPUTE DP = PROBIT(HIP) - PROBIT(FPP). COMPUTE BETA = EXP(-0.5*(PROBIT(HIP)**(2))) / EXP(-0.5*(PROBIT(FPP)**(2))). LIST VARIABLES= DP BETA HI FN FP CN.
* Multiple invocations of the macro in the same interactive session will produce the following note:
>The macro name specified on the DEFINE command duplicates the name of a previously defined macro. This instance will take precedence.
This warning does not indicate a problem and may be ignored.