Trend questions | Types of questions | ACT Science

A trend is a pattern in a graph or table of data. On the ACT, trends are often used to relate two different variables. Trend questions are very common in the Science section, so you should be comfortable spotting and describing them.

Identifying trends

First, you’ll learn how to recognize trends in tables and graphs. Then you’ll see how to use a trend to make a prediction.

Trends in tables

When you look for trends in tables, you usually compare two columns. Even if a table has many columns, the question will typically ask you to relate just two of them. Here’s a simple example using columns $x$ and $y$ :

$x$	$y$
1	5
2	10
3	15
4	20
5	25

As the $x$ values increase, the $y$ values also increase. That’s the trend in this table.

If that feels straightforward, try describing the trend in the next example.

Temperature (º)	Density (m³/kg)	Viscosity (mm²/s)	Vapor Pressure (kPa)
15	1.004	1.3	1.74
20	1.000	1.1	2.35
25	0.998	1.0	3.16
30	0.994	0.8	4.19
35	0.990	0.5	5.52

As temperature increases, what happens to the viscosity of the chemicals?

Give it a try, then check the explanation.

(spoiler)

As temperature increases (from $15$ to $35$ ), viscosity decreases (from $1.3$ to $0.5$ ).

Even though the table has several columns, you only need the two mentioned in the question: temperature and viscosity. To spot the trend, move down the table and watch how one column changes as the other increases.

Trends in graphs

To find a trend in a graph, look at the line (or the pattern of points) and describe how it moves as you go from left to right.

As the $x$ -axis values increase, does the line go up or down?
In other words, do the $y$ values increase or decrease?

Example science trend chart

In this graph, the line rises as you move to the right. That means as $x$ increases, $y$ increases. In context, you could say: “As the distance from the epicenter increases, the time difference between P and S waves also increases.”

If a graph has multiple lines, they may not follow the same trend. One line might increase while another decreases. Make sure you’re describing the trend for the specific line the question asks about.

Using trends as prediction

Some questions ask you to estimate a value that isn’t shown directly, based on the trend. For example, the graph above ends at $x = 400$ km. A question might ask you to predict the time difference between P and S waves at $x = 500$ km. There are a few ways to do this.

First, you can use a quick (but not perfectly precise) method: extend the line visually. In other words, sketch how the line would continue past where it stops, following the same general direction and steepness.

Second, you can estimate using the most recent change in the graph. Here, you want the value $100$ km after the graph ends, so look at how much the line changes over the last $100$ km shown (from $300$ to $400$ ). Here’s the graph again:

Example annotated science trend chart

From $x = 300$ to $x = 400$ (a change of $100$ km), the line increases by about $15$ seconds on the $y$ -axis, from $47$ to $62$ (shown by the blue lines). Use that same increase to estimate the value at $x = 500$ km:

Start from the last known point: $62$
Add the estimated increase over the next $100$ km: $62 + 15 = 77$

No matter which method you use, the goal is the same: extend the trend to the $x$ value mentioned in the question and estimate the corresponding $y$ value.

Key points

Trends. A trend is a pattern in a table or graph. When one variable changes, what happens to the other variables?

Tables. You can notice trends in tables by looking at what happens when you move up or down the table. In other words, what happens when one variable increases or decreases?

Graphs. Track trends on graphs by looking at the axes. When you look farther down one axis, do the points get farther on the other axis too?

Predictions. You can imagine how a graph or a table might continue if you understand the trend. On a graph, you might simply trace the line past the grid to the value in question. On a table, you might add rows until you reach the value in question. Assume the trend will not change outside the figure!