Measuring Poverty (9): Absolute and Relative Poverty Lines

There are many ways you can measure how many people in a country are poor. Quite common is the use of a so-called poverty line. First you decide what you mean by poverty – for instance an income that’s insufficient to buy life’s necessities, or an income that’s less than half the average income etc. Then you calculate your poverty line – for instance the amount of income someone needs in order to buy necessities, or the income that’s half the average income, or the income of the person who has the tenth lowest income if the population was one hundred etc. And then you just select the people who are under this poverty line.

I intentionally chose these examples to make a point about absolute and relative poverty. In the U.S., people mostly use an absolute poverty line, whereas in Europe relative poverty lines are used as well. As is clear from the examples above, an absolute poverty line is a threshold, usually expressed in terms of income that is sufficient for basic needs, that is fixed over time in real terms. In other words, it’s adjusted for inflation only and doesn’t move with economic growth, average income, changes in living standards or needs.

A relative poverty line, on the other hand, varies with income growth or economic growth, usually 1-to-1 since it’s commonly expressed as a fixed percentage of average or median income. (It obviously can have an elasticity of less than 1 since you may want to avoid a disproportionate impact on the poverty line of very high and very volatile incomes. I’ve never heard of an elasticity of more than 1).

Both absolute and relative poverty lines can be criticized. Does an absolute poverty line make sense when we know that expectations change, that basic needs change (in contemporary Western societies, not having a car, a phone or a bank account can lead to poverty), and that the things that you need to fully participate in society are a lot different now than they once were? We know that people’s well-being does not only depend on the avoidance of absolute deprivation but also on comparisons with others. The average standard of living defines people’s expectations and when they are unable to reach the average, they feel excluded, powerless and resentful. Can people who fail to realize their own expectations, who lose their self-esteem, and who feel excluded and marginalized be called “poor”? Probably yes. They are, in a sense, deprived. It all depends which definition of poverty we can agree on.

It seems that people do think about poverty in this relative sense. If you compare the (rarely used) relative poverty line of 50% of median income in the U.S. with the so-called subjective poverty lines that result from regular Gallup polls asking Americans “how much they would need to get along”, you’ll see that the lines correspond quite well.

So if relative poverty corresponds to common sense, it seems to be a good measure. On the other hand, a relative poverty line means moving the goal posts for all eternity. We’ll never vanquish relative poverty since this type of poverty just moves as incomes rise. It’s even the case that relative poverty can increase as absolute poverty decreases, namely when there’s strong economic growth (i.e. strong average income growth) combined with widening income inequality (something we’ve seen for example in the U.S. during the last decades). (Technically, if you use the median earner as the benchmark, relative poverty can disappear if all earners who are below the median earner move towards the median and earn just $1 or so less than the median. But in practice I don’t see that happening).

Lies, Damned Lies, and Statistics (21): Misleading Averages

Did you hear the joke about the statistician who put her head in the oven and her feet in the refrigerator? She said, “On average, I feel just fine.” That’s the same message as in this more widely known joke about statisticians drowning in a pond with an average depth of 3ft. And then there’s this one: did you know that the great majority of people have more than the average number of legs? It’s obvious, really: Among the 57 million people in Britain, there are probably 5,000 people who have only one leg. Therefore, the average number of legs is less than 2. In this case, the median would be a better measure than the average or the mean.

But seriously now, averages can be very misleading, also in statistical work in the field of human rights. Take income data, for example. Income as such isn’t a human rights issue, but poverty is. When we look at income data, we may see that average income is rising. However, this may be due to extreme increases at the top 1% of income. If you then exclude the income increases of the top 1% of the population, the large majority of people may not experience rising income. Possible even the opposite. And rising average income – even excluding extremes at the top levels – is perfectly compatible with rising poverty for certain parts of the population.

Averages are often skewed by outliers. That is why it’s necessary to remove outliers and calculate the averages without them. That will give you a better picture of the characteristics of the general population (the “real” average income evolution in my example). A simple way to neutralize outliers is to look at the median – the middle value of a series of values – rather than the average (or the mean).

An average (or a median for that matter) also doesn’t say anything about the extremes (or, in stat-speak, about the variability or dispersion of the population). A high average income level can hide extremely low and high income levels for certain parts of the population. So, for example, if you start to compare income levels across different countries, you’ll use the average income. Yet country A may have a lower average income than country B, but also lower levels of poverty than country B. That’s because the dispersion of income levels in country A is much smaller than in country B. The average in B is the result of adding together extremely low incomes (i.e. poverty) and extremely high incomes, whereas the average in A comes from the sum of incomes that are much more equal. From the point of view of poverty average income is misleading because it identifies country A as most poor, whereas in reality there are more poor people in country B. So when looking at averages, it’s always good to look at the standard deviation as well. SD is a measure of the dispersion around the mean.

More posts in this series.

The Causes of Poverty (24): Population Growth and Income Growth: Incompatible?

Some blame overpopulation for many of the world’s problems such as poverty, famine and war (which are obviously rights violations). There are supposed to be too many people for peaceful coexistence and sustainable food production. Those who worry about overpopulation are often called (neo-)Malthusians, and either predict a sharp fall in population levels because of the problems caused by overpopulation (a “Malthusian catastrophe”), or/and propose population control as a measure to solve these problems.

For pretty much all of human history, population growth constrained growth in real standards of living. That’s the “Malthusian Trap”: as standards of living improved, population increased, which put a strain on resources and drove down standards of living, which in turn drove down population growth, rinse & repeat. The industrial revolution broke this trap, although it’s worth pointing out the fairly obvious fact that this is not true for the entire world. Conor Clarke (source)

… over a roughly 3000 year period, during which there was obviously quite a lot of technological progress — iron plows, horse collars, mastering the cultivation of rice, the importation of potatoes into Europe, etc. — living standards basically went nowhere. Why? Because population growth always ate up the gains, pushing living standards back to roughly subsistence.

… technological change was slow — so slow that by 1600 or so, when England had finally reclaimed its population losses from the Black Death, it found real wages back to more or less 1300 levels again.

And here’s the sense in which Malthus was right: he had a fundamentally valid model of the pre-Industrial Revolution economy, which was one in which technological progress translated into more people, not higher living standards. This homeostasis only broke down when very rapid technological change finally outstripped population pressure for an extended period. Paul Krugman (source)

It’s clear that population growth can go hand in hand with income growth, and that it’s not correct to state that population growth necessarily leads to more poverty, which in turn leads to a reversal of population growth. But these compatible evolutions of population and income seem to require technological advances.

Note: my criticism of Malthusianism and other types of overpopulation hysteria (see here for some examples) is targeted only at deterministic theories which believe in overpopulation as the main if not only cause for the world’s problems, and which see overpopulation as a global problem. I accept that in certain specific areas of the world, population pressures can make things worse. But I don’t agree that these pressures are the sole or even the main cause of problems such as poverty, famine, war etc. And neither do I agree that population control is the main remedy for these problems. For example, we all know that water shortages – even very local ones – aren’t caused by overpopulation and won’t be solved by population control. More intelligent irrigation methods are the answer. And when we leave the local level and take the global point of view, the population problem is even less salient. On a world scale, income has grown systematically faster than the world’s population during the last centuries. Population pressures do not lead us to an inevitable “trap” as Malthus and his followers claim.