Case Identification

Table 1. Table 1. Characteristics of Shelby County, Tennessee, and King County, Washington, as of 1980.*

Shelby County, Tennessee, and King County, Washington, are the most populous counties in their states, containing the cities of Memphis and Seattle, respectively. Although both counties are urban in character, King County is predominantly white and enjoys a relatively high standard of living. In contrast, Shelby County has a large black minority, and a substantial proportion of its citizens live below the poverty level (Table 1).22 , 23

All suicides involving a resident of either county that occurred between August 23, 1987, and April 30, 1990, were studied to identify those that took place in the home of the victim. Any death ruled a suicide was included, regardless of the method used. Self-inflicted injuries that were not immediately fatal were included if death followed within three months. Cases in which there was potential litigation over the official cause of death were excluded at the request of the medical examiners.

Case Selection and Recruitment of Case Proxies

A home was defined as any house, apartment, or dwelling occupied by a victim (a case subject) as that person's principal residence. Suicides occurring in adjacent structures (e.g., a garage) or the surrounding yard were also included. In the event of a murder—suicide, the suicide victim was included only if older than the murder victim; in double suicides, only the older victim was included.

Reports made at the scene were collected to ensure that the study criteria were met. In King County, the medical examiner's staff conducted all investigations of the scene. In Shelby County, police officers conducted the investigations. In addition to data obtained at the scene, investigators obtained the names of persons close to the victim who might provide an interview at a later date. These lists were supplemented with names obtained from newspaper accounts, obituaries, and calls to funeral homes. In this way, proxies for the victims were identified.

Approximately three weeks after each death, those chosen to be interviewed were sent a signed letter that outlined the nature of the project. A $10 incentive was offered, and a follow-up telephone call was made a few days later to arrange a time and place for the interview. At the time of this meeting, informed consent was obtained. The proxies were told that they could decline to answer any question. They were also free to terminate the interview at any time.

Selection and Recruitment of Controls

After each interview with a case proxy, we sought a control subject matched with the case subject according to sex, race, age range (0 to 14, 15 to 24, 25 to 40, 41 to 60, and 61 years or older), and neighborhood of residence. To minimize selection bias, the controls were identified by a previously validated procedure for the random selection of matching households in nearby locations.24 , 25 After marking off a one-block avoidance zone around the home of the case subject, the interviewers started a neighborhood census at a randomly selected point along a predetermined route radiating out from the case subject's residence. Households in which no one was home were visited twice more, at different times of day and on different days of the week. After the census was completed, an adult (18 years old or older) in the first household in which a member met the matching criteria was offered a $10 incentive and asked to consent to an interview. Whenever possible, attempts were made to interview someone other than the actual matching control subject (a control proxy). When no interview was granted, the next matching household on the route was approached.

Interviews

The interviews with proxies for case subjects and their controls were virtually identical in format, order, and content. Each was brief, highly structured, and arranged so that the more sensitive questions were not broached until later in the interview. Items drawn from the Short Michigan Alcoholism Screening Test,26 the Hollingshead two-factor index of social position,27 and a 1978 poll by Decision Making Information28 were included. Particularly sensitive questions were preceded by "permissive" statements, such as the following: "Half of all homes in America contain one or more firearms. Are guns of any kind kept in your home?" To confirm the reliability of the interviewers, supervisory personnel contacted a random sample of 10 percent of those interviewed and administered an abbreviated form of the questionnaire. The rate of concordance between these responses and those given to the interviewers equaled or exceeded 96 percent.

Statistical Analysis

Reports from the scene of the suicide were analyzed for descriptive purposes only. Interview data were used in the risk assessment because these were collected in an analogous manner from the case proxies and the controls. Since any member of a control household might decide to remove firearms from the home after a death in the neighborhood, the answers were adjusted to reflect the state of affairs on the date of each suicide. MantelHaenszel chi-square analysis was used to calculate the crude odds ratio associated with each variable. Subsequent multivariate analyses used conditional logistic regression, the appropriate technique for a matched-pairs design.29

Potentially confounding variables were identified and controlled for through a three-step process. First, models containing closely related variables (such as those describing the use of alcohol in the home) were constructed to identify the variable or variables in each set that were most predictive of being a case subject or a control. Whenever both measures of exposure pertaining to the individual and measures pertaining to the household were found to be significant, only the variable describing exposure for the individual was retained. Next, a model that incorporated the variables selected in this initial step was constructed, to select the variables that remained significant after we controlled for the effects of the remaining variables in the model. Finally, a third model was constructed in which gun ownership was added to the list of covariates to assess the relation between firearms and suicide in the home after adjustment for the effects of the remaining variables. At this stage, we also looked for two-way interaction terms and checked to see whether there was an interaction between gun ownership and any of the matching variables.

After calculating an adjusted odds ratio for suicide based on all the pairs for which data were complete (360 pairs), we analyzed a series of subgroups representing various strata in the full study sample. To exclude the possibility of confounding due to preexisting mental illness or depression, we calculated the relative odds of suicide among case subjects who had no history of psychiatric problems. To limit bias resulting from potentially faulty reporting about case subjects who lived alone, we conducted an analysis restricted to pairs in which interviews were obtained with proxies who lived in the same home as the victim. To determine whether gun ownership was associated with an increased risk of suicide by means other than firearms, an analysis stratified according to the method of suicide was also conducted.

After these calculations, we performed three final analyses to see whether different patterns of gun ownership or storage were associated with a greater or lesser risk of suicide. In each comparison we used the final model as previously defined, but we subdivided gun ownership into three categories instead of comparing owners with nonowners. One analysis compared handgun ownership, ownership only of long guns, and ownership of no guns. A second compared ownership of loaded guns, ownership of unloaded guns, and ownership of no guns. The third compared guns kept in a locked place, guns kept in an unlocked place, and no guns.