How many beers will a teacher's salary buy? Whether it is hamburgers, beauty products, fizzy drinks or the frequently used Mars bar index, it is interesting to see how well teachers are paid around the world. This week's map compares the income of an average teacher with that of the average worker in each country. Of course, an average salary covers the range between the lowest-paid new entrant and the highest-paid school leader, but it provides evidence of the relative worth of teachers in a society.
The striking feature of the map is that in many countries the average teacher earns just slightly more than the average wage. Whether this represents a good return on the extra education teachers need for their jobs compared with many workers is an interesting question.
Certainly, where the cost of that investment is borne by an individual, rather than the state, it is logical to expect teachers to demand a return on the cash they have expended - for instance, on tuition fees. This may be why across the US teacher salaries have to be high enough to allow teachers to service their student debt in order the enter the profession.
There is a message here for British policymakers who would increase tuition fees. Even if tuition is free, or fees are low, a long period of study without an income, as in Germany, will require a return in salary terms to attract teachers into the profession.
Finally, developing countries keen to expand their economic influence in the world are likely to value teachers as a key resource for economic growth. This may explain the high relative salaries in Thailand and Mexico.
The salaries shown on the map represent the situation at a time of relative economic prosperity. What will be the position in a few years' time, when job security may overtake relative salary as a keen concern for many? The focus may be on bankers' bonuses, but it is teachers' salaries that may suffer in an age of austerity.
John Howson is director of Education Data Surveys, part of TSL Education.