State the single overall aim clearly. It is often useful to express this as a question:

• How many …?
• Is there a relationship between ...?
• What are …?
• How much …?
• Why do …?

or you may use phrases such as:

• Patterns of …
• An evaluation of ...
• A literature review of
• Secondary analysis of ...
• An assessment of ...

Do not include methods: your aim is not, for example, "to carry out a survey" but to summarise the overall purpose of doing so.