In general, having multiple items will increase your reliability of measurement. A common measure of what the literature calls "subjective well-being" is a combination of the following scales.
- The PANAS: A measure of positive and negative affectivity. A 20 item measure of positive and negative affect (see Watson et al, 1988).
- Diener's Life Satisfaction Scale. Five short items measuring life satisfaction.
I.e., well-being is measured as a combination of high positive affect, low negative affect, and high life satisfaction. Of course, there are many other measures of subjective well-being (for a review, see Kahneman and Krueger, 2006).
When to use single item and when to use multiple item
In general there are trade-offs in research. More items brings greater reliability but it also takes more time, which can potentially reduce participation or lead to other costs.
A single item measure of well-being might be appropriate when:
- measuring the construct reliably is not that important
- You are concerned with population level estimates and you have large sample sizes per group
- You are doing experience sampling research where you are getting tens or hundreds of observations of well-being at different time points, and thus, participant fatigue is a major issue.
In your case, you are conducting a pre-post study to assess the effect of an intervention. In such cases, it would really be worth administering a reliable multi-item measure of well-being. For example, to complete the PANAS plus Diener Life Satisfaction combination above, might only take a couple of minutes. For the sake of getting reliable measurement, that seems like a valuable use of two minutes.
That said, if you still wish to explore single item measures, check out this previous answer on where Gaël Laurans mentions Russel et al 1989 and Larsen et al 2009 as useful sources:
- Watson, D., Clark, L. A., & Tellegen, A. (1988). Development and validation of brief measures of positive and negative affect: The PANAS scales. Journal of Personality and Social Psychology, 54(6), 1063-1070.
- Kahneman, D. & Krueger, A.B. (2006). Developments in the measurement of subjective well-being. The journal of economic perspectives, 20, 3-24. PDF
- Russell, J.A., Weiss, A., & Mendelsohn, G.A. (1989). Affect Grid: A Single-Item Scale of Pleasure and Arousal. Journal of Personality and Social Psychology, 57 (3), 493-502.
- Larsen, J.T., Norris, C.J., McGraw, A.P., Hawkley, L.C., & Cacioppo, J.T. (2009). The evaluative space grid: A single-item measure of positivity and negativity. Cognition and Emotion, 23 (3), 453-480.