## What to blame for misconceptions in mathematics?

Oliver Knill, December 19, 2015

 A study detects that half of a test group of college students can not decide correctly whether 1/5 is larger than 1/8. Psychologists attribute this to memorization techniques used by the students. A different interpretation is that students fail to give the right answer simply because they lack any pre-wired techniques to solve such basic problems reliably. It is possible that it is not the memorization which is the problem, but that it is the lack of mastering of firm algorithmic structures and the presence of gaps in knowledge foundations, which is the root of the problem.

One of the students in my Math E320 Extension school class mentioned this study: The interpretation of that "psychology today" article is that students have problems to compare 1/5 with 1/8 because "they have memorized the algorithm of cross multiplication."

I believe that the source of the problem that many students do not dare to cross multiply because they have been instructed to hold off standard techniques. The generic technique of cross multiplication would give and prove the right answer. While also simple ``common sense" thinking: [if you divide a cake into 5 pieces or 8 pieces, in which case are the pieces larger ] gives the solution, common sense will fail for more complex problems.

[December 25 added: here is a beautiful example from here: you take 60 stairs when going up an escalator and 90 when returning down the same stairs, how many stairs would you take if the stairs would not move?]. It is not only remarkable about this example appears in a popular journal in Germany; it should also be noted also that the problem can be solved by using pre-wired techniques which one learns as a mathematician. If you think that shortcuts work, then no, the obvious answer 75 is totally wrong! You have to learn techniques to think in order to solve such problems reliably. And this needs memorization as it requires to know the technique!

Mathematics gives us the power to solve problems which go beyond simple intuition and which also allows us to make sure that the thought process is correct and the statement is true and not just a lucky guess.

Intuition and argumentation is important, but in more abstract concepts already present in basic geometry or probability, common sense can fool us (Monty Hall, Geometric proofs that all triangles are equilateral etc). Mathematics is not politics, where fuzzy thinking, sloppy definitions or rhetorical tricks rule.

Having solid thinking and procedural problems skills is one of the main goals in math education. Why is it true that 1/5 is larger than 1/8? It is because a simple cross multiplication (or producing a common denominator) renders this equivalent to 8 is larger than 5. The algorithm of cross multiplication is not the devil. It not only solves this particular problem, it also proves the statement. Here is a maybe provocative claim:

 The demonisation of learning "procedures" and acquiring "knowledge" done by some psychologists the last few decades is one of the main reasons for poor testing outcomes. Indeed, there is a cultural war on "Knowledge" and "Algorithmic thinking". It can be summarized in the statement ``you shal not memorize" or ``apply prewired algorithms".

When we work with students on calculus problems, it is staggering to observe that often, basic arithmetic skills like cross multiplication working with fractions are not solidified. It is as if some inner voice tells the students, not to do anything automatic. They feel helpless and insecure.

Maybe the following recommendation is more reasonable:

 Building solid knowledge and algorithmic skills, which at first are used blindly by applying patterns gives the confidence and hunger for a deeper understanding. It is the soil,* on which creativity can flourish later.

* yes, sometimes the soil is dirty and unattractive.