Comparing two charts is quite common in IELTS writing task 1. Sometimes the charts might be of the same items but different years or of different items. It is even possible to have three charts.
First off, get to analyze the data presented and whether there are years or not. Looking at the years is important because it dictates the tense you are to use in the response.
You then start with the introduction, overall statement and the description like any other chart is described. Sometimes test takers write the overall statement at the end, which is quite okay.
The difference now comes in the manner of describing and comparing the data in the main body. This means your identification of the data and comparing it necessitates using adjectives and comparative language.
You can start by comparing the main or highest data of the charts, then the rest. Look at the following example.
TASK RESPONSE..
The two pie chars describe the time spent on smartphones and tablets for different category usage.
Ideally, both smartphones and tablets are used mainly for games, with news being the lest used in percentage for smartphones compared to utilities for tablets.
A detailed analysis shows that games account for an enormous percentage for both, at 57% for tablets, a bit higher than smartphones for which 35% is used. Social networking, though quite lower than games, takes a sizeable percentage for smartphones and tablets, 29% and 15% respectively. Though utilities take the third position, just over a third, at 20% with smartphones, music and videos account for 13% with tablets, being the third highest. For smartphones, music caries as much time as “others” for tablets at 8%, being among the minimally used category for both.
The least categories for smartphones were news and others, all below 5%, just like utilities and news for tablets.
153 WORDS.
I want you to note that I compare the data for both at the same time. At times test takers will write one paragraph describing the data of the first pie chart and then another describing the data of the second pie chart. In this way, this is a normal description rather than a comparison.