Exam terms

Countercurrent flow

Countercurrent exchange is a mechanism occurring in fish gills and lots of other places Biology, such as the loop of Henle in kidney nephrons. It’s also used industry and engineering in such things as heat exchangers, in which there is a crossover of some property, usually heat or some chemical, between two flowing bodies flowing …

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This term is used to describe the relationship or link between two variables. If an increase in one variable is matched by an increase in the other variable (the line points up) then there is a positive correlation. If the line points down it is a negative correlation. If the data points are scattered it …

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Control variable

A control variable is an element that is not changed throughout an experiment because its unchanging state allows the relationship between the other variables being tested to be better understood. Be careful not to confuse the different types of variable in an experiment. A useful way to remember the difference is that your results depend …

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Continuous variable

A continuous variable is a variable that has an infinite number of possible values. In other words, any value is possible for the variable. A continuous variable is the opposite of a discrete or a categoric variable, which can only take on a certain number of values.


In the context of the exam questions, this means that: your answers should be written in the space provided. For example, on a diagram in spaces in a sentence  or in a table. Examiners mark on a computer screen that repeatedly shows only one part of the same question at a time. We mark the …

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In the context of the exam questions, this means that: you are required to describe the similarities and/or differences between things,  not just write about one.  It is a very common mistake in exams to consider only the characteristics of one thing when this command word is used in the question. For a valid comparison …

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Categoric variable

Categoric variables have values that are labels, eg names of plants or types of material. Categoric variables are also often known as discrete variables, but categoric variables is the term preferred by AQA examiners.


Marking a scale on a measuring instrument.  This involves establishing the relationship between indications of a measuring instrument and standard or reference quantity values, which must be applied. Such as  placing a thermometer in melting ice to see whether it reads zero in order to check if it has been calibrated correctly.