User Note - Injury and Poisoning Supplement Variables
- I. Background
- II. Episode-level Injury and Poisoning Variables
- III. Creating the Person-level Injury and Poisoning Variables
- IV. Variables Available
- V. Take Note: Comparability Issues
- VI. Take Note: Generating Estimates from the Injury and Poisoning Data
Beginning in 1997, the NHIS has included injury and poisoning supplements to gather information about medically attended or consulted injuries and poisonings occurring for any member of the family within a three-month reference period. An injury episode refers to a traumatic event in which a person was injured one or more times from an external cause (e.g., a fall, a motor vehicle traffic accident). An injury condition is the acute condition or the physical harm caused by the traumatic event. Likewise, a poisoning episode refers to the traumatic event resulting from ingestion of or contact with harmful substances, as well as overdoses or incorrect use of any drug or medication, while a poisoning is the acute condition or the physical harm caused by the traumatic event. Each episode has at least one injury condition or poisoning classified according to the nature-of-injury codes 800-999 in the Ninth Revision of the International Classification of Diseases (ICD-9-CM).
The family respondent provided data about injury and poisoning episodes for the family. From 1997 to 1999, NHIS released separate injury or poisoning episode-level data files, which provide information for each injury episode, and person-level data files, which aggregate this episode-level data to the person who experienced each injury and/or poisoning episodes. The person-level injury or poisoning file contains the number of times a person was injured or poisoned by specific causes, the number of times a person was injured or poisoned while doing specific activities, the number of times a person was injured or poisoned in specific places, and the number of injury or poisoning episodes a person has in the episode-level file.
Beginning in 2000, NHIS only released episode level data for injuries and poisonings. Additionally, rather than having separate injury and poisoning files, episodes for injuries and poisonings are combined into a single, episode-level file from 2000 forward. IPUMS NHIS has used this episode-level data to generate person-level data that is comparable to the person-level data offered in 1997-1999.
Additionally, IPUMS NHIS offers the episode-level data used to create the person-level injury and poisoning variables. Users who wish to include more detailed information at the level of injury or poisoning episodes may wish to use these hierarchical injury variables. Episode-level data are only available through IPUMS NHIS hierarchical extracts, which allow users to select episode-level variables as well as the person- and household-level variables always available through the IPUMS NHIS extract system; the variables are not available in the rectangular extract format. Users can rectangularize these hierarchical extracts using the IPUMS NHIS-constructed unique identifier NHISIID. NHISIID uniquely identifies any injury record across all samples. The first 16 columns of NHISIID are identical to NHISPID, which uniquely identifies all persons across all samples, and the first 12 columns of NHISIID are comparable to NHISHID, which uniquely identifies all households across all samples. These keys can be used to identify the person to whom injury records belong and the households to which those persons belong. The remaining digits of NHISIID indicate the episode number and if it was an injury or poisoning epsisode (for 1997-1999). For more information on using our extract system, see our User Support Page, which includes multiple helpful tutorials.
II. Episode-level Injury and Poisoning Variables
Separate Injury and Poisoning Files, 1997-1999
In 1997-1999, questions about poisoning episodes were asked about separately from injury episodes; injury and poisoning data were released in two separate files in these years. To improve comparability with the 2000-forward files (where injury and poisoning episodes are offered in a single file), IPUMS NHIS has combined the poisoning and injury episode data for 1997-1999 into a single record type.
The structure of codes in the combined IPUMS NHIS injury and poisoning records mimics that implemented by NHIS/NCHS in 2000-forward. Injury episodes are classified as "NIU" or "not in universe" for poisoning-specific variables; and similarly poisoning episodes are "NIU" for injury-specific variables. Variables that are relevant to both types of episodes (e.g., time since episode occurred, hospitalized as a result of the episode, etc.) have observations for both injury and poisoning records. Episodes that were originally released in the poisoning file in 1997-1999 can be by identified by the variable IRPOISYN. IRPOISYN also denotes poisoning episodes from the combined injury and poisoning files in 2000-forward surveys to help users easily identify poisoning episodes.
III. Creating the Person-level Injury and Poisoning Variables
The IPUMS NHIS person-level injury and poisoning records for 2000 forward were created using the episode-level data. IPUMS NHIS replicated the person-level variables available in 1997-1999 samples. Each person-level variable is generated by summing the number of unique times the family respondent reported a particular cause of injury, location at the time of injury, and activity at the time of injury. The resulting person-level variables report the total number of times a person had an injury or poisoning episode as a result of each given cause, at each given location, and while doing each given activity.
IV. Variables Available
Summary variables include whether or not the person had an injury or poisoning episode in the past three months that was serious enough that she/he sought medical advice or treatment (INJURY3MO), the number of injury or poisoning episodes the person experienced in the past 3 months (INJURY3MONO), and the total number of conditions resulting from the person's injury and poisoning episode(s) (INJCONDITION). Beginning in 2004, the total number of conditions resulting from the injury and poisoning episode(s) are restricted by conditions that can be classified as injuries or poisonings according the ICD-9-CM diagnostic codes. Prior to 2000, poisonings were asked about separately from injuries. Data users should see POISON3MO and POISON3MONO, which report if the person experienced a poisoning that caused them to seek medical advice or treatment (including calls to the poison control center) in the past 3 months, and the number of times a person experienced such a poisoning. Beginning in 2000, poisoning is instead provided as a cause of injury because poisonings are not asked about separately from injuries.
Variables related to the causes of injuries include transportation (INJCAUSTRAN), a fire/burn/scald (INJCAUSBURN), a fall (INJCAUSFALL), a poisoning (INJCAUSPOIN), overexertion or strenuous movements (INJCAUSOVER), being struck by an object or person (INJCAUSTRIK), an animal or insect bite (INJCAUSANIM), a cut or piercing (INJCAUSCUT), machinery (INJCAUSMACH), or some other cause (INJCAUSOTH). INJCAUSPOIN is not available prior to 2000 as poisonings were asked about separately in these years. The cause of injury is determined according to ICD-9-CM external cause codes. Specific codes for each year are listed here: 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, and 2013, 2014. Beginning in 2015, ICD-10-CM external cause codes are used instead. Additional information about 2015-forward codes are listed here: 2015, and 2016.
Variables that describe the location of the person at the time of injury or poisoning include inside the home (INJPLAINHOM), outside of the home (INJPLAOUTHOM), at a non-residential school (INJPLASCHOL), at a child care center or preschool (INJPLADAYCAR), at a residential institution other than a hospital (INJPLARESID), at a health care facility including hospitals (INJPLAHEALTH), on a street or highway (INJPLASTREET), on a sidewalk (INJPLASIDE), at a sports facility, athletic field, or playground (INJPLASPORT), at a shopping center, restaurant, bank, gas station, or other place of business (INJPLATRADE), on a farm (INJPLAFARM), at a park or recreation area including biking and jogging paths (INJPLAPARK), at a river, lake, stream, ocean, or other body of water (INJPLARIVER), at a swimming pool (INJPLAPOOL), at an industrial or construction area (INJPLACONST), at a mine or quarry (INJPLAMINE), at some other public building (INJPLAPUBLIC), and some other location not specified in the response categories (INJPLAOTH). There are slight changes in the wording of the locations included as response categories, but they do not change the underlying meaning of each response category and are therefore comparable across time. Not all response categories are available in all samples. The locations "swimming pool" and "mine or quarry" are not offered as response categories after 2000. Sidewalk was first included as a response category in 2004. While these changes in response categories may affect the frequency of "other" responses as well as the most the response categories most closely related to eliminated or added categories, possible locations at the time of injury were removed based on the declining frequency of responses.
Variables that describe the activity of the person at the time of injury or poisoning include driving or riding in a motor vehicle (INJDODRIVE), working at a paid job (INJDOPAID), working around the house or yard (INJDOHOM), attending school (INJDOSCHOL), doing unpaid or volunteer work (INJDOUNPAID), doing sports or exercise (INJDOSPORT), doing a leisure activity other than sports (INJDOLEISUR), sleeping, resting, eating, or drinking (INJDOREST), cooking (INJDOCOOK), being cared for (INJDOCARED), or some other activity (INJDOTH). There are slight changes in the wording for the activities that a person was doing at the time of the injury or poisoning, but they do not change the underlying meaning of each response category and are therefore comparable across time.
V. Take Note: Comparability Issues
While individual variable descriptions will note comparability issues specific to each variable, there are several cross-cutting comparability issues that users should understand before using the injury and poisoning data.
Between 1997 and 1999 NHIS only collected detailed information on the four most recent injury or poisoning episodes. Starting in 2000, a person may have up to ten injury and/or poisoning episodes; a person appears in the injury and poisoning episode-level file for each unique injury and/or poisoning episode that she/he experienced. As such, users should exercise caution when comparing person-level frequencies in the pre-2000 and post-2000 periods.
Persons reported episodes that they considered poisonings (e.g., food poisoning and allergic reactions) but that are not considered poisonings based on the ICD-9-CM. These types of episodes were included in the 1997-2003 episode-level data files. Beginning in 2004, episodes that are not considered poisoning episodes based on ICD-9-CM are no longer included in the injury/poisoning data files. Since these episodes are not poisoning episodes according to the ICD-9-CM, the IPUMS NHIS variable INJCONDITION does not include these episodes when estimating the number of conditions from injury and poisoning episodes. Data users should be aware that these episodes may affect the comparability of frequencies in episode-level analyses using pre-2004 and post-2004 samples.
Additionally, between the 2003 and 2004 surveys, there was a change to the handling of the recall period for injury and poisoning episodes. Persons are asked about injury or poisoning episodes that occurred in the past 3 months and then asked for the date of each episode among other questions. In 1997-2003, the data files include episodes that were reported to occur within 104 days or 4 months of the interview as well as episodes where the date of the injury or poisoning was not reported. Beginning in 2004, episodes that were reported to have occurred within 91 days or 3 months of the interview based on the family questionnaire responses were retained. If the date of the episode was subsequently reported as outside of the reference period, the episodes were still included in the files. The IPUMS NHIS variables IRDAYSLB and IRDAYSUB report the upper and lower bounds of the recoded number of days between the episode and the interview, and can be used to determine if reported episodes may have occurred outside of the 91 day reference period.
In 2000, a key question was inadvertently reworded. The question asking respondents "How many different times in the past three months were you injured or poisoned seriously enough to seek medical advice or treatment?" was mistakenly changed to "How many times in the past three months did you seek medical advice because you were injured or poisoned?" As a result, respondents gave the number of times they sought medical advice for an injury or poisoning during the three months prior to the interview, rather than the number of times they were injured or poisoned during the three months prior to the interview. In 2001, this wording was reversed to the original question wording "How many different times in the past three months were you injured or poisoned seriously enough to seek medical advice or treatment?"
Users are strongly cautioned that any national incidence estimates derived from the 2000 NHIS injury and poisoning episode data will be underestimates. Despite the significant decline in the overall number of injury and poisoning episodes in 2000, the percentage distributions of episodes for many injury-related variables are comparable across time, such as the external cause of the injury, the nature of the injury, the injured person's activity at the time of injury, where the injury happened (e.g., inside the home, outside the home, at work, etc.) and any limitation of activity resulting from the injury.
VI. Take Note: Generating Estimates from the Injury and Poisoning Data
Questions in the Injury/Poisoning section of the NHIS have a recall period of the "last 3 months." However, as the time between the injury/poisoning episode and the date the injury/poisoning questions were asked increases, the annualized number of injuries/poisonings reported decreases. For most analyses of the injury/poisoning data (e.g., estimates for all types of injury/poisoning episodes and estimates for less severe injuries/poisonings), limiting data to episodes with five weeks or fewer between the injury/poisoning episode and the date the injury/poisoning questions were asked is recommended because analyses showed that respondents tend to forget less serious injuries (Warner, et al., 2005). For analysis of injury/poisoning episodes resulting in more serious outcomes (e.g., estimates for fractures and hospitalizations) that are unlikely to be forgotten, the data does not need to be limited to the five-week period. The longer period of time between the injury/poisoning episode and the date the injury/poisoning questions were asked will increase the number of episodes reported and therefore increase the size of the sample and provide richer detail and greater stability in the estimate.
To calculate an annual estimate of the number of injuries and poisonings, multiply the weighted number of episodes reported during a time period by the number of time periods in a year. For instance, to estimate the number of injury or poisoning episodes occurring annually using episodes with three months or less elapsing between the injury/poisoning and the date the injury/poisoning questions were asked, each three-month weighted count should be multiplied by 4 (i.e., by 12/3=4). If users choose to limit their analysis to episodes within five weeks or fewer between the injury/poisoning and the date the injury/poisoning questions were asked, each five-week weighted count should be multiplied by 10.4 (i.e., 52/5=10.4). If users choose to calculate two different estimates, one for serious and one for non-serious injuries/poisonings, they are advised NOT to combine the two estimates.
Analysts are cautioned against estimating the number of different people injured or poisoned annually using the current NHIS questions. Estimating the number of persons injured using the annualizing method described in the above paragraph (i.e., multiplying the estimate by the number of time periods in a year) assumes that the same individuals experienced injuries at the same rate over the year. Analysts are cautioned to closely read NHIS documentation and review the specific item in the questionnaire in order to insure that annual estimates for these kinds of injury or poisoning episodes have intrinsic meaning.