Presenting information in a way that clearly answers interesting questions is challenging. Every plot has an implicit question (hypothesis) that it helps you answer. Therefore, it is important to align a visual display of information with the intended interesting question(s). Collaboration or consultation with a statistician can clarify interesting questions and lead to answers through appropriate data analysis.
Suicide was the topic of the front cover story in the Daily Lobo last Thursday. With the story, two pie charts displayed average annual proportions of successful and unsuccessful suicides by method in New Mexico.
The successful pie chart answers this statement of conditional probability: “Given a successful suicide, what percentage used certain methods?” A question I consider more interesting, however, reverses the conditioning: “Given an attempted suicide with a certain method, what percentage were successful?” Furthermore, I want to know the overall frequency and percentage of each method attempted. How can we present the information in a way that simultaneously answers these questions?
The Suicide Prevention Resource Center (SPRC.org) maintains national and state suicide fact sheets, last updated September 2008, describing “deaths by suicide, estimated hospitalized attempts, and data on medical costs, work loss costs, gender, race/ethnicity, age and method of suicide.” The pie charts in Thursday’s Daily Lobo were reproductions of those found on the New Mexico fact sheet. From their summaries, below is the SPRC table for estimated mean frequencies by method for successful and unsuccessful suicides.
Their question and pie charts consider percentages down columns.
When the data are reduced to row percentages for successful and unsuccessful attempts separately, you lose the relative frequency of attempts. The percentage of firearms successes (56 percent), for example, depends on all the other successful attempts. Because proportions for successful and unsuccessful attempts are separate, you can’t learn about how successful firearm attempts are.
There is a temporal process: a person first chooses a method, then makes an attempt and is either successful or not. The data display and questions should follow these temporal steps. The pie chart displays ignore this process.
My question and plot considers the temporal process of attempting suicide, considering percentages across rows, including row total information. First, the relative use of various methods is clear: almost two-thirds of attempts are by poisoning, and firearm and cut/pierce are each just above one-in-10. But even though attempts by firearms (12 percent) and cut/pierce (13 percent) are relatively rare, the success rates are extremely different (92 percent vs. 2 percent)! The plot has been sorted by the numbers of successes to emphasize the relative risk of the methods in terms of lives, information which is lost in the pie charts.
The Agora Crisis Center (505-277-3013, 9 a.m. to midnight, every day) plays a critical role in our community, and our education as individuals around these issues can save someone. Using statistics and visualization to tell and understand the important story in the data can lead to improvements in strategies and resource allocation for treatment and prevention.
Erik B. Erhardt is an assistant professor of statistics and is the UNM statistics and consulting clinic director. He can be reached at firstname.lastname@example.org