Reading an Experiment

Before reading any numbers, find the one or two things being deliberately changed and the thing being measured. Here the scientists change the diet and the amount of seaweed supplement, and they measure the gas each cow produces. Everything else is deliberately held the same: one herd, groups of the same size, the same two-week period. That fixing is the whole point, because if the groups differed in some other way you could not tell whether the supplement or that other difference produced the result.

In a two-way table each cell is one combination, so you need both the row and the column. High supplement with low forage gives thirty grams, while no supplement with the same diet gives a hundred and fifty. The important habit is to read down a column when you want the supplement's effect, because the diet stays fixed as you go down. Reading across a row changes the diet instead and answers a completely different question — and the options are written so that both readings give a number you could pick.

Now a claim: does the supplement always reduce the production of both gases? Methane falls down every column — a hundred and fifty to forty to thirty, two hundred and forty down to thirty, two hundred and forty down to a hundred. Always. Carbon dioxide does not: in the low-forage column it actually rises from seven thousand six hundred to seven thousand eight hundred. One exception is enough, so the answer is methane only. To test the word always you must check every cell, and the fastest way is to hunt for a counter-example rather than confirm all nine.

A results table usually gives an average per animal per day, so getting a group total takes two more multiplications. A hundred grams per cow per day, times ten cows, is a thousand grams a day for the group, and times seven days is seven thousand grams for the week. The trap is that the options include the numbers you get by stopping early — a thousand if you forget the week, seven hundred if you forget the cows. Saying the unit out loud at each step is what keeps you honest: grams per cow per day, then per group per day, then per group per week.

The hardest item in a block like this asks why the study was run a particular way, and there is no arithmetic in it at all. Why measure only during the second week? Ask what would go wrong otherwise: measuring in the first week would catch cows whose digestion was still adjusting to a new supplement, so the readings would not have settled. That is the answer. Options offering convenience — fewer measurements — or good intentions — reducing emissions — are never the reason a scientist designs something this way. Look for the flaw being avoided. Quiz and exercises now.