Mode
The mode is the value that occurs most often in a set of observations, useful for counted or categorical lab data.
Definition
The mode is the measurement that shows up most often in your data, the most common result. It is most useful for discrete counts and categorical observations, where a mean may not make sense, and a set can have one mode, several, or none; for continuous measurements like time or mass it is rarely useful because repeated trials seldom give exactly the same value. For continuous data the mode is found by binning values into a histogram and locating the tallest bar, so it depends on the bin width chosen. A bimodal distribution signals that the data come from two populations or two conditions, and the mode is the statistic that makes this visible when mean and median cannot.
Example
Counting leaves on eight bean plants gives $4$, $5$, $5$, $6$, $5$, $4$, $5$, $6$, so the mode is $5$ leaves. Recording the eye color of 24 classmates as brown $13$, blue $7$, green $3$, hazel $1$, the mode is brown, and a mean eye color would be meaningless. Cart roll distances from 30 trials binned in $5\ \text{cm}$ intervals show peaks at $80$ to $85\ \text{cm}$ and $95$ to $100\ \text{cm}$; investigation reveals that one student released the cart with a trigger and another by hand, an uncontrolled variable that a single mean of $89\ \text{cm}$ would have concealed.
Key Insight
The mode is the only average that works for things that are not numbers, like the most common flower color in a garden. Two modes often mean two different groups are mixed together, such as seeds from two packets, so the mode can reveal structure the mean hides. Mean, median, and mode agree for a symmetric bell-shaped distribution; when they disagree, the shape of the data is telling you something, and a good lab report says what.