Google Trends includes a function that allows users to
compare up to five terms, locations or time ranges. Where this function
is used, Google Trends generates a line graph with multiple data lines
on it, as well as a bar graph.
Google Trends will also
generate a set of "regional interest" tables. However, the scores contained
within these tables are not modified as a result of the comparison. In
other words, they are the same as if no comparison had been undertake.
it is not possible to utilise more than one type of comparison
simultaneously. Therefore, a comparison of time ranges and a comparison of
locations is restricted to a single term.
How Are Scores Calculated?
Comparison of Search Terms:
Scores generated in a comparison of search terms are relative to one another.
In order to obtain these relative scores, Google Trends compares relative search volume for each term with specified time range.
If for example, "Search Term A" accounted for 0.0001% of total search queries on 10th January and Search Term B represented 0.00005% of total search queries on the same day, the scores awarded to A and B would be equivalent to 2 - 1. The scaling of the scores between 0 and 100 would depend on how the the relative search volume for both terms on the day compares to the highest relative search volume within the time range.
The scores contained within the bar graph represent the average score for the terms over the specified time range.
When a comparison of search terms is undertaken with "global' parameters, the relative search volume that is compared is based on an average derived from all the national relative search volumes.
Read more about comparisons of search terms here: Google Trends - Comparison of Terms.
Comparison of Time Ranges:
comparison of time ranges calculates scores in a very similar way to a comparison of terms (described above).
Google Trends calculates the relative search volume for the terms across the different time ranges and then collectively scales the data between 0 and 100.
Comparison of locations:
comparison between locations generates scores calculated in the same
way that "regional interest" scores are calculated. The scores are
calculated by analysing:
(1) the popularity of the search term relative to total search within the specific territories
(2) the relative popularity of the search term within each specific territory, relative to the other territories.
For more general information on "regional interest" scores read Understanding Google Trends.