AP Biology · Lesson 7 of 7 · Unit 2

Graphing data, error bars and statistical significance

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Data representation and statistical analysis appear across every unit on the AP Biology exam, testing Science Practice 4 (Representing and Describing Data) and Science Practice 5 (Statistical Tests and Data Analysis). Most notably, Free-Response Question 2 Part B requires you to construct a graph by hand in your paper answer booklet worth 4 points—more than 10% of the entire free-response section.

By the end you’ll be able to

Construct an appropriate graph (line, bar, scatter plot, histogram, box-and-whisker, log scale or dual y-axis) from a data table.
Label axes with variables and units, add a legend, choose appropriate scaling and plot data accurately, including error bars and a trend line when appropriate.
Describe data by identifying specific data points, trends and patterns, and relationships between variables.

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Core ideas (continued)
Worked examples
Exam technique
Common mistakes
Quick recap
Check your understanding (3 questions)

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What the exam asks

Section I (Multiple Choice): Stimulus sets regularly display bar graphs, line graphs, scatter plots, box-and-whisker plots, dual $y$ -axis graphs, or log-scale graphs. You must describe trends, calculate rates, means, or standard error of the mean ( $SE_{\bar{x}}$ ), and determine whether treatment groups differ statistically by inspecting $\pm 2\,SE_{\bar{x}}$ error bars.
Section II (Free Response):
- Question 2 (Long FRQ, 9 points): Part B (4 points) always requires you to construct a graph on the provided grid from a data table, including error bars when given. Part C (2 points) often asks you to analyze the data, calculate a mean or rate, or use error bars to evaluate a null hypothesis.
- Question 1 (Long FRQ) & Question 6 (Short FRQ, Analyze Data): Ask you to describe specific data points or overall trends, and evaluate a hypothesis or claim using the graph’s means and error bars.

Core ideas

Choosing the right graph type

Data structure	Graph type	Example in Unit 2 (Cells)
Continuous independent variable ( $x$ -axis) showing change over a continuous range	Line graph or scatter plot (with a line of best fit / trend line if testing correlation)	Percent change in potato core mass across sucrose molarities ( $0.0\text{ M}$ to $1.0\text{ M}$ )
Categorical / discrete independent variable ( $x$ -axis)	Bar graph (with spaces between bars)	Rate of aquaporin-mediated water transport in wild-type vs. mutant cells

What the 4 points on FRQ 2 Part B reward

Each question’s own scoring guideline decides how its 4 points are split, so do not count on a fixed one-point-per-feature pattern; in some questions, one of the 4 points is for using the graph or its error bars to draw a conclusion. These are the features the graph points reward:

Orientation and labels: Independent variable on the $x$ -axis, dependent variable on the $y$ -axis, and both axes labeled with units (borrowed directly from the data table headers).
Scaling: Linear (or appropriate log) scale with uniform intervals that spread your data across most of the grid. Do not simply copy irregular table numbers onto grid lines.
Plotting and graph type: Correct graph type (bar vs. line/scatter), all means accurately plotted, and a clear key/legend if multiple data series are plotted.
Error bars / trend line: When the data give error values, accurately plotted error bars ( $\bar{x} + 2\,SE_{\bar{x}}$ and ) above and below each mean; an appropriate trend line when requested.