6.2 The Study: Weight-Loss-Related Self-Efficacy

6.2.1 Study Information Description

Weight-loss interventions aim to support people in changing their behaviours to be healthier. This includes promoting healthy-eating and exercise. Self-efficacy is thought to be important for adopting and maintaining such behaviours. That is, people will change their behaviours if they believe they can maintain those behaviours and if they believe that changing their behaviour will help them lose weight.

Because self-efficacy is so important to the success of weight-loss interventions, some such interventions now include psychotherapy specifically aimed at improving self-efficacy. Weight-loss interventions provide instrumental support in helping participants develop a plan for eating healthful foods and engaging in regular physical activity. This has a clear and obvious benefit for self-efficacy. The current study will examine how subsequent psychotherapy further boosts self-efficacy. Hypothesis

The primary hypothesis is that adding psychotherapy to a weight-loss intervention will lead to increased weight-loss self-efficacy.

6.2.2 Design Plan Study Design

This study will use a within-subjects (paired) design. Participants will be overweight or obese individuals recruited from the community and enrolled in a weight-loss intervention at UBCO. They will attend a group session where they are provided general information about healthy eating, and physical activity. They will then have one-on-one interviews with a nutritionist to help them build meal plans that fit their budgets and lifestyles. Following the interviews, the nutritionist will ask participants to complete measures of weight-loss self-efficacy (WLSE). Participants will then meet with a trained psychotherapist who will work with the participants to identify potential barriers to their success and develop strategies for overcoming those barriers. After psychotherapy, participants will be asked to complete the same measure of WLSE. Measures

Weight-loss-related behaviour self-efficacy will be measured using the scale proposed by Wilson et al. (2016). This scale has four items, measured on an 11-point scale ranging from 0% (Not at All Confident) to 100% (Completely Confident). The scale is relatively new; however, it has demonstrated strong psychometric properties in a large sample drawn from a population that is similar to the population used in the current study (Wilson et al., 2016). Analytic Strategy

We will conduct a directional paired samples t test of the null hypothesis that average change in WLSE in the population is less than or equal to zero. Significance will be based on the traditional \(\alpha\) = .05. Sample Size and Power Analysis

We want to be able to detect a difference of 10 points on the WLSE measure. This measure had a SD = 21.44 in a large sample from a similar population (Wilson et al., 2016). Assuming the same SD in our sample, a 10-point change translates to Cohen’s dz = 0.47. To achieve 95% power with a population effect of that size requires a sample of N = 50.4. We are therefore aiming for a sample of N = 51.


Wilson, K. E., Harden, S. M., Almeida, F. A., You, W., Hill, J. L., Goessl, C., & Estabrooks, P. A. (2016). Brief self-efficacy scales for use in weight-loss trials: Preliminary evidence of validity. Psychological Assessment, 28(10), 1255–1264. https://doi.org/10.1037/pas0000249