Inverting a Logistic Function

I was recently talking to a researcher who had conducted a cognitive experiment that involved experimentally manipulating a variable x, a continuous property of a stimulus, looking at the effect on a variable p, the probability of giving a response. The function p(x) was assumed to be a logistic function. The researcher wanted to know how to calculate the point on x at which the fitted logistic regression function equalled 0.5.

Fitting Nonlinear Regression Models to Multiple Participants Using SPSS

This post briefly discusses how to run a nonlinear regression in SPSS. Specifically, it discusses the scenario where you have a a set of k observations for each of n participants, and where your aim is to fit a nonlinear function to the data of each participant in order to save the parameter estimates for subsequent analysis. This is a relatively common task in psychology. You have multiple participants measured on a numeric repeated measures variable and you want to see how a dependent variable is related to this repeated measures variable. And you want to do this separately for each participant. For example, you might be modelling performance as a function of practice or accuracy as a function of stimulus intensity.

Issues in Model Building and Parameter Estimation | Case Studies in Psychology

In this post I discuss issues related to Model Testing and Parameter Estimation. I focus on the role of this process in the scientific development of knowledge. I was motivated to write this post in an attempt to integrate the set of modelling issues that I was encountering across a range of psychological research topics including psychological tests, learning curves, social networks, and well-being. The post provides links to additional resources and presents some of my own observations on model testing and parameter estimation. I make the disclaimer that my ideas are still evolving on this topic.