One would think that literacy (ability to read) would improve ones chances to work at better paying jobs, so a positive correlation would result. That is the conclusion of the web link I provided. I do not claim it to be impartial, as it is from a website that promotes literacy. I don't have know of a study that concludes there is a negative correlation. If illiteracy percentages are calculated and then compared to per capita GDP for particular country, a negative correlation could result.
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Strong and positive
Income is a ratio measure. In ratio measures, one can order categories, specify the difference between two categories, and the value of zero on the variable represents the absence of the variable. Thus, income can take on values of $0, $10, $30,000, etc. Zero dollar income means the absence of income, making income a ratio measurement.
Possibly... It depends on how you code income. If income is listed as an actual amount of money earned within a year, then it is interval data because there is a measurable difference between the income of one person vs the income of a second person. For example, one person might make $25,000 and another might make $26,000. We know that the first person makes exactly $1,000 less than person 2. We also know that person 2 makes more than person 1. If I have 10 people and I call the richest person "1" the second richest person "2", etc. Then the numbers 1 - 10 are ordinal data for "income". Notice that Ordinal data does not give us any measurable distance between two people. Income "5" might be $1 more than income "6". But Income 7 might be $10,000 less than income "6". All we know with ordinal data is that 1 is larger than 2, 2 is larger than 3, etc.
Income is quantitative
You calculate the income for each household. This is the sum of the incomes of all members of the household. The median is the value of household income such that 50% of households have a higher income and 50% have a lower income.