Interpolation means estimating a value between plotted points; extrapolation means extending the trend beyond the data—both show up on ACT Science when a question asks for a value the graph does not label directly. Follow the trend line or table pattern; do not guess randomly. Child of ACT Science calculating questions.
Interpolation (between points)
Example: the graph plots temperature at 0 s, 10 s, and 20 s. The question asks for 15 s. Find 15 on the x-axis, follow up to the line or bar, read the y-value. For straight-line segments, the midpoint is often the average of the two neighboring values.
Extrapolation (beyond points)
Example: data ends at 20 s but the question asks what would happen at 30 s if the trend continues. Extend the visible pattern—linear rise, plateau, decline—only as far as the graph supports. If the trend curves or changes slope, note that extrapolation is less certain; pick the answer that matches the direction, not a precise number unless the line is clearly linear.
When to use each
- Interpolation: the asked value falls inside the x-axis range shown
- Extrapolation:the asked value is outside the plotted range but the question says "if the trend continues"
Connect to graph reading
These questions reward the same axis-first habit as every other Science graph item. Brief intro in how to read graphs faster on ACT Science; trend language in interpreting trends.
Estimate to eliminate
Even when you interpolate, cross out answers that are off by an order of magnitude or contradict the trend direction. For exact ratios from table values, see the main process in calculating questions.
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