Computing dprimes and betas

Dprimes and Betas may be computed in SPSS using the probit function. The UCLA statistics department give [http://www.linguistics.ucla.edu/faciliti/facilities/statistics/dprime.htm further details.] They suggest values upto 2 are typical dprime values.

A table showing dprime values for true positives and false alarm combinations.[:FAQ/sigdet/effecttab: is available.]

• Also available by typing sigdet at a UNIX prompt

• Excel syntax available [http://psy.ucsd.edu/~kang/sdt/sdt.htm 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.