An illustration of the resource vs coverage problem

The illustration below is taken from an old book, I’m reading (*):

Fig. 3 shows a desert land which will be cultivated by irrigation, i.e. the artificial application of water. Fig. 4 shows the amount of fertile soil available. Now, the farmer can decide to spread the soil all over the area, by which the layer of fertile soil will be so thin that nothing will grow in any part of the land. That is not a good plan and all the work involved will be fruitless.
But there’s an alternative: The soil can be spread over a section of the land, for example the area marked in fig. 3. This way the layer of soil will be thick enough to ensure that there will be exuberant growth and good utilization in the smaller area. This is obviously a much better plan.
This scenario not only applies to farming: It illustrates a problem we often face in testing, where the amount of functionality being developed is much larger than the what we can cover in a decent way. It is my experience that it is always better to focus testing on sections of the system than to try to check everything: There will be areas of the system which will be left untested, but what you test, you will cover well.
As a decision maker, I’d much rather have in depth knowledge about parts of the system, than to know very little about everyting. It will give me much better foundation for making good business decisions.
*) The book is Holger Paaskesen: “Vi lærer for livet?”  from 1968. It’s English title would be “We learn for life?” and it’s a book about school education.