Is That Chart Telling You the Truth? Five Questions for Any Health Graph

Jocelyn Michel

7 minutes ago

3 min read

When researchers checked the graphs in drug ads aimed at doctors, more than a third of them didn't match the drug's own numbers. A picture reaches us before our thinking does. That's what makes it so persuasive, and so easy to bend.

The good news is that checking a chart takes seconds once you know where to look. Here are five questions to ask any health graph, with examples so you can see each one for yourself.

Question 1: Where does the line start?

Look at the numbers up the side of the graph. If they don't start at zero, a small change can look like a mountain.

In the example below, the numbers are exactly the same in both charts. The only difference is where the side begins.

Two line charts showing the same numbers. The left chart’s side starts at zero and the line rises gently. The right chart’s side starts higher, so the same rise looks steep.

Not every chart needs to start at zero. A change in body temperature, for example, would disappear on a chart that started at zero. The point is to notice where it starts, and ask whether the picture matches the size of the change.

Question 2: What's along the bottom?

Check the dates or categories along the bottom. Are they in order? How long a stretch of time is shown? A short window, or days placed out of order, can tell a completely different story.

Two bar charts of the same seven days of headache scores. On the left, the days are in order and the headaches get worse. On the right, the days are sorted by size, so they look like they’re getting better.

Question 3: Does the size match the number?

When a chart uses pictures or shapes instead of plain bars, our eyes read the whole area, not just the height. Something drawn twice as tall and twice as wide looks four times bigger, even if the number only doubled.

On the left, two bars show $30 and $60, with the second bar twice as tall. On the right, the same amounts are drawn as circles, and the second circle looks four times as big.

Question 4: What's missing?

Look for labels, numbers and a source. Ask:

- What do the sides of the chart measure?

- Where did the numbers come from?

- How many people are behind them?

An honest chart wants you to check it. A chart with nothing to check deserves a harder look.

Question 5: Who made it, and what do they want me to feel?

Hopeful? Afraid? Ready to buy? Notice the feeling first. Then go back and look at the numbers. When a company that sells a product shows you a dramatic chart, slow down. It doesn't mean they're lying. It means the picture was designed to sell.

Pictures can tell the truth, too

In 1858, Florence Nightingale drew a diagram showing that most British soldiers who died in the Crimean War died of preventable disease, not their wounds. It helped persuade the government to clean up its military hospitals. The same tool that can hide the truth can reveal it.

Your own honest chart

The most trustworthy chart in your life may be one you draw yourself. If you keep a daily log of your pain, sleep, energy or mood, you can turn it into a simple picture:

- Put the days along the bottom, in order.

- Put your score from 0 to 10 up the side, starting at zero.

- Mark one dot for each day.

- Draw a line on the day you started something new.

An example daily pain chart over fourteen days, scored from zero to ten, with a dashed line where new care began and pain gradually going down.

No one is selling you anything with this chart. Bring it to your next appointment, and your practitioner can see in one glance what might take twenty minutes to explain.

A practice for today

Find one chart or graph about health this week: a news story, an ad, a social media post or a label. Ask the five questions. Then write one line: "This picture is honest," "This picture is stretched," or "I can't tell." Each answer is information.

Words to Carry

- A graph is an argument, not a fact.

- I check where the line starts.

- I check what's along the bottom.

- I ask whether the size matches the number.

- I notice what the picture wants me to feel.

With you every step of the way,

Dr. Jocelyn

References

Cooper, R. J., Schriger, D. L., Wallace, R. C., Mikulich, V. J., & Wilkes, M. S. (2003). The quantity and quality of scientific graphs in pharmaceutical advertisements. Journal of General Internal Medicine, 18(4), 294–297.

Huff, D. (1954). How to lie with statistics. W. W. Norton.

National Army Museum. (n.d.). Florence Nightingale: The lady with the lamp. https://www.nam.ac.uk/explore/florence-nightingale-lady-lamp