Two plus two equals four? Easy.

How about this: You're supposed to take a medication either one hour before a meal or two hours after.  It's noon, and you've just started lunch.  When should you take the medication?

About 40 percent of participants in one U.S. study could not figure that out.

They would be termed innumerate — unskilled at working with numbers.  As opposed to numerate, or mathematically adept.

Ellen Peters, a visiting scholar at the Annenberg Public Policy Center at the University of Pennsylvania who also did her undergrad work at Penn, has been exploring the world of innumeracy and its effects on health care, among other things.  She's writing a book, Innumeracy in the Wild.

A distinguished professor of psychology at Ohio State, Peters spoke to us recently about numeracy and health care.

 Tell us a little more about numeracy and innumeracy. What do they mean?

Numeracy is like literacy for numbers. Having started as an engineer, I saw that people often didn't use numbers in their decisions when I thought they should. Numeric information does a lot for us.  It gives meaning to all kinds of things we do: shopping, getting a particular medical treatment, evaluating an environmental risk, making a financial decision.

But not everyone can understand and use numbers effectively.  It turns out that a lot more people have adequate literacy than have adequate numeracy.  One study found that while 99 percent of participants had adequate literacy, only 17 percent had better than ninth-grade numeracy skills.

According to data reported in 2013, almost a third of the U.S. adult population can do only simple operations with numbers — they can count, sort, and do simple arithmetic. If you translate that into the current U.S. adult population, there are about 73 million people who can do up to only basic problems with numbers.

Why does this matter in the realm of health care?

Researchers have started to find that people who are less numerate have worse health. It turns out they take on fewer protective health behaviors. They might not realize how susceptible they are to some disease or how much some positive health behaviors might help them. In Ghana, for example, we found that people who were less numerate were less likely to use a condom to prevent HIV. Lower numeracy is associated with more diseases, more medication. People who are less numerate end up in hospitals and emergency rooms more often.

People with lower numeracy also understand less in some important ways. About 40 percent of Americans cannot figure out when to take a missed medication dose, given instructions on their prescription bottle. That's important because, according to some studies, 70 percent of adult Americans take at least one medication a day. About 50 percent take two or more a day.

They also understand risks and benefits of medical treatments less well. That kind of comprehension is critical, though. It's the first, early building block of making a good decision.

And numeracy goes beyond comprehension to how you process information, and how you integrate numbers with other information. People who are lower in numeracy tend to rely on concrete and easy-to-evaluate things.  Those might be a story a friend told them, an anecdote they read in the newspaper.  Imagine you're a male trying to decide whether to get prostate cancer treatment, and you heard from a friend that it did or didn't work for him.  That might make a bigger difference to you than the numeric risks of the treatment. Someone who is highly numerate might use the story somewhat, but they'll depend more on the numbers.

Let’s look more closely at a specific example: diabetes.

Diabetes is probably one of the most highly numeric diseases we have.  Only about 9 percent of American adults can do all of the numeric tasks that are needed to manage diabetes effectively.

What do you have to do if you're diabetic? You have to know how to accurately understand and use the readings from glucose meters. You have to be able to modify your insulin dosage based on those glucose levels. But the dosage also has to be based on your level of physical activity, food portion sizes, carbohydrate content. To do that, you have to be able to pull information from food labels.

People with diabetes also deal with health insurance a lot. To figure out how much money to put into a health savings account, they have to be able to estimate total annual costs.  They have to count up everything and estimate how much insulin they'll need, how many test strips they'll need for the glucose meters, and so much more.

Without adequate numeracy skills to manage the disease, you can't control your blood sugar levels effectively. You can get sicker.

What’s another example of numeracy affecting the outcome of a disease?

One study that looked at adult patients hospitalized with a cardiac problem found that 59 percent of them misunderstood why they were taking cardiac medicine, or how much or how often they were supposed to take it.  The patients who weren't good with numbers misunderstood more often.

Another study found that people who are less numerate perceive less value in their medication. To really value something, you have to understand how likely you are to get the disease if you do — or don't — take the medicine. Those who are numerate have a gist for the magnitude of what the medication does, how big the risk is, and how much lower it gets if you take the medication.

If patients know they have limited numeracy, what should they do?

Patients can ask some key questions to get at information that maybe their physician doesn't think about.

For one, imagine your doctor told you: "You might get a headache from this medication." You can ask, "Well, how likely is that in terms of the numbers?" This may seem counterintuitive for people who are innumerate.  But in one study we did, people were far more likely to overestimate the chances of a side effect and not be willing to take the medication when told headaches were "common" than when told that 7 percent of people get them. This was true whether the person was more numerate or less numerate. So, asking for the numbers can help.

Also, you might be told that a particular drug will double your risk of some scary side effect. That's called relative risk. But you can also ask about the absolute risk. It could be that you're doubling your risk from 20 percent to 40 percent. That's huge.  Or it could be that you are doubling it from 0.01 percent to 0.02 percent. That's tiny.

Another thing you can do: Sometimes there are just too many treatments to consider. If you're a man considering treatment for prostate cancer, you might have four or five options. It can be very difficult to evaluate them, especially given the emotional state you're in.  You can tell your doctor, "Can you simplify this? Can you give me the best two options to consider?"