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---------- From: Andrew Booth Sent: 20 August 2001 15:38 To: [log in to unmask] Subject: Fixed effects versus random effects models - idiot's guide? Does anyone have a simple explanation or "rule of thumb" for explaining to people when a fixed effects or a random effects method should be used for a systematic review. Although I have had the technicalities explained to me at an excellent systematic reviews course I would like a simple way to explain it to others when facilitating critical appraisal. Also the review we looked at: Towler et al. BMJ 1998 317: 559-563 on Hemoccult for colorectal cancer screening used both models. Is this a "belt and braces" approach or are there sound indications for when you should use both models together? Yours hopefully Andrew Booth