Evaluate Wiles' study of the effectiveness of CBT.

One major strength of Wiles' study is its strong methodology. It was a randomised controlled trial (RCT), which is considered the 'gold standard' in research for establishing cause and effect. By randomly allocating participants to either the CBT group or the control group, it minimises the impact of participant variables, meaning we can be more confident that it was the CBT that caused the improvement, not some other factor. The study also had a large sample size (469 participants), which increases the generalisability of the findings to other people with treatment-resistant depression. Another strength is the study's practical application. The findings provide clear evidence that CBT is a valuable treatment option for patients who do not respond to antidepressants. This has important implications for clinical practice, suggesting that a combination of treatments may be the most effective approach and giving doctors and patients another evidence-based option to try. However, a potential weakness is the reliance on self-report measures. Depression was measured using the Beck Depression Inventory (BDI), which requires participants to rate their own symptoms. This can be subject to social desirability bias, where participants may downplay their symptoms to appear better, or their responses may not accurately reflect their true state. An objective measure, in addition to the self-report, would have made the results more reliable.