Literacy, health, and per capita income are the most often used criteria for comparing countries. Literacy rates are high in almost all countries, so this criterion is not very useful for country comparison. Health rates vary greatly from country to country, so they can't be used to compare countries. Per capita income is the most accurate way to compare countries because it takes account of each country's population size. It is also the most relevant factor since a higher income means more choice and better care. In addition to these three common methods, other techniques have been developed over time by statisticians to compare countries further. For example, some use data on life expectancy or education levels instead of literacy rates. The two main methods used for comparing countries are called "comparative analysis" and "correlation analysis". These terms are used here to describe any method that compares two things by taking account of how much they resemble each other. In comparative analysis, each country is given a score between 0 and 1, with 0 being no connection at all and 1 being a perfect match. Then the scores for all countries are added up and divided by the number of countries studied.