The purpose of retrospectives is to plan ways to increase quality and effectiveness.  How this is done is up to the team to manage. Unless patterns are changed then teams can become disengaged with the continuous improvement focus that retrospectives are intended to provide.
While the Scrum framework is not prescriptive in how Sprint Retrospectives are conducted, there are some common patterns.
Team members should review what actions or behaviours they had previously committed to. When experiments had been defined, there may be the opportunity to review results.
Team members can generate ideas on how to improve quality or effectiveness. This is commonly the only part of a Sprint Retrospective that teams play close attention to. The idea generation process can be changed from sprint to sprint, so as to alter patterns of thinking and generate new ideas.
There are many different ways to generate improvement ideas beyond the standard questions of “what worked”, “what didn’t work”, etc. 
Team members can devise experiments for improvements or actions to take with a focus on improving quality and/or effectiveness. These may consist of concrete actions or behaviours to encourage.
It is recommended to standardise the idea capture/experimentation process but introduce different ways to generate improvement ideas. This helps from the process from becoming stale and keeping team members engaged.
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