Scenario 3 – Ideal Portion Sizes

As a behavioural scientist, you have been commissioned to carry out a study for a ready-meal company to determine an ideal 'everyday portion size' for one of their products. You decide to use the Method of Constant Stimuli in a discrimination experiment where participants are shown a picture of a food portion on a computer screen. The portion size changes over a series of trials and the participant is asked to indicate whether the portion is larger or smaller than their normal portion size. The pictures you show are of a popular dish (curry with rice) and have been carefully measured in terms of their calorific content (kcal). Each food stimulus is displayed thirty times.

A typical set of data for one participant is shown here:

Size of food portion (kcal)	50	100	150	200	250	300	350	400
Number of responses regarded as larger than their normal portion	4	7	11	20	24	27	28	30

Plot a psychometric function from this data.

What is the standard stimulus in this experiment?

Estimate the PSE.

Estimate the difference threshold.

Briefly explain what these two values represent in the context of this experiment. What portion size would you recommend to the company?