Scientific method and quantitative biology definitions

Variables and controls: what an experiment can show

Independent variable

The quantity you change on purpose, and the only one that should differ between your treatments. It goes on the horizontal axis.

Dependent variable

The quantity you measure, whose value depends on the independent variable. It goes on the vertical axis.

Control variable

A quantity held the same across every treatment, so that it cannot be the reason the dependent variable changed.

Control experiment

An extra run of the method, differing in one deliberate way, done to rule out an explanation the main runs cannot rule out on their own.

Confounding variable

A variable that changes with the independent variable and could itself explain the change in the dependent variable, so the two effects cannot be separated by the data collected.

Randomisation

Assigning subjects or sample positions by chance rather than by choice, so that variables not identified in advance tend to balance out between the groups on average.

Correlation

A relationship in which two variables tend to change together. It may be positive or negative, strong or weak, and it makes no claim about which variable moved first.

Causation

A relationship in which changing one variable changes the other. It cannot be read off a scatter graph.

Valid

A measurement is valid if it measures what it claims to measure; an investigation is valid if its design allows its conclusion to follow. A confounding variable destroys validity however careful the measuring was.

Accurate

Close to the true value.

Precise

Repeat readings close to one another. Precision says nothing at all about being right.

Repeatable

The same person, using the same method and the same apparatus, gets the same results again.

Reproducible

Somebody else, or the same person using different apparatus or a different method, gets the same results.

Resolution (of an instrument)

The smallest change an instrument can register: 0.01 g on a two-decimal-place balance, 1 mm on a metre rule.

Hazard

Anything with the potential to cause harm, such as a chemical, a sharp edge or a pathogen.

Risk

How likely it is that a hazard causes harm, and how serious the harm would be, under the conditions in which the work is done. Control measures reduce the risk; the hazard stays the same.

Sentience

The capacity to have experiences that feel good or bad to the animal itself, such as pain, distress or comfort.

The 3Rs

Replacement, Reduction and Refinement: replace animals with other methods where possible, reduce the number used, and refine procedures so that the animals used suffer as little as possible. Set out by Russell and Burch in 1959.

Handling data: units, scales, rates and uncertainty

Significant figures

The digits in a measurement that carry information about its size, counted from the first non-zero digit.

Leading zeros

Never significant. 0.0043 has two significant figures, because the zeros only locate the decimal point.

Trailing zeros after a decimal point

Always significant. 4.20 has three, and says the hundredths digit was measured.

Averages, spread and the null hypothesis

Mean

The total divided by the number of readings. Uses every value, and is what almost every biological calculation wants.

Median

The middle value when the readings are put in order. Unmoved by a single extreme value, which is why it is used for skewed data such as incomes, reaction times or the number of parasites per host.

Mode

The most common value. The only one of the three that works for categories such as blood group or flower colour.

Null hypothesis

A statement that there is no significant difference, no significant association or no significant correlation between the variables, and that any difference observed is due to chance.

Significance level

The probability threshold, conventionally 0.05, below which the null hypothesis is rejected.

Degrees of freedom

A number, worked out from the number of categories for chi-squared or from the sample sizes for a t-test, that decides which critical value you read from the table.