What sampling technique is this?

Language Track Changes Mark-up Options Reviewing Accept Resolve Show Comments Au Section 3. Methods In this study, comparing 6 countries, (3 countries with a low and low - middle GDP and 3 with a high GDP) and calculating the countries with a lower GDP before and after the rollout, opposed to the countries with a higher GDP and evaluating how their economies have boosted respectively. The countries include; 3 countries with low to medium - low GDP per Capita and 3 counties with high GDP per Capita and see how much of an impact the vaccine has on their economies. The sampling technique was