The base refers to everyone that's included in your analysis. Think of it as your reference point.
By default, the base includes everyone that's represented by the dataset you've got selected (e.g. all internet users aged 16-64 for GWI Core), but sometimes you may want something more specific. Here're three common examples of when you might consider applying a base.
1. When some people just aren't relevant to your analysis
Let's say you're doing some analysis for a beauty brand who're launching a skincare product targeted at men, and have created an audience of "male internet users who’ve used 3 or more skincare products in the last week and want brands to be eco-friendly".
Here, you could apply a base of men to help you understand what makes your audience different from other male internet users.
Note: Because you're updating your point of reference, your indexes will refer to the "average male internet user aged 16-64" rather than "the average internet user aged 16-64".
2. When a question has been routed
We don’t like wasting people’s time, which is why we don’t ask our respondents irrelevant questions. For example, in GWI Core, we only ask respondents who own a vehicle about fuel type. This is known as routing.
When using the fuel type question, you may therefore want to apply a base of vehicle owners so you're only looking at those respondents who were asked the question.
Hint: Click the three little dots next to a data point in charts, select "view question" then click "notes" to find out more about the question and see whether any routing has been used.
3. When using a data point from a recontact module or extension study
We use multiple surveys to capture a broad range of data about internet users. Let's take a look at GWI Core as an example:
- All respondents take either the main survey or mobile survey
- Some of these respondents then complete the brand & media recontact module
- Some of these respondents then take part in our extension studies (e.g. GWI Core Plus)
All of these components are weighted to represent the same overall universe of 16-64 year olds, and can therefore be used together. This means you could be looking at something from the main survey, and then decide to pull in something from the brand and media module, and then something from GWI Core Plus.
This gives you loads of flexibility, but it also means you may have multiple bases active within the same chart or crosstab. This isn’t a problem. In fact, it’s how we recommend you keep things as it means each data point and relationship is being calculated using the largest number of respondents possible. But if consistency is your priority, you can filter out anyone who didn’t answer all of the questions you’re using by applying a base of respondents who took part in the relevant recontact module or extension study. For GWI Core Plus, this can be found in the "Survey Details" folder under the name of "Audience Size".