Hi all,

I am doing some dose-response experiments and want to use probit

analysis to calculate the 50th percentile values. When I use the probit

analysis routine of SPSS 9, I get weird results. Here is a sample set of

data:

Concentration number of subjects observed responses

0.00 78 2

0.06 49 4

0.13 61 14

0.18 114 68

0.20 99 78

0.23 105 79

0.25 82 77

0.50 89 89

The output from the probit analysis gives an intercept of -2.26864 and a

slope of 14.22993. The graph of the regression line shows all probit

transformed values to lie between -2 and 1.8. This line and the data

points on the graph do not make any sense to me. My data should convert

to probit values between 3 and 7+. What am I doing wrong? How do I get a

printout of the actual probit values used by SPSS?

Thanks,

Tony Haigh