Data management is a major task in most intervention trials. The main stages of the
data management process are:
- entering the data into a computer system
- checking the data for errors and inconsistencies
- organizing the data into an appropriate form for analysis
- archiving the data.
Good data management requires a well-defined data management strategy—it is not something that will just happen. The complexity of the data management process will depend on the size and type of study. Attention to data management will greatly reduce the time needed in the analysis stage, because the data will be well organized and consistent and have fewer errors.
5.1 Data entry
The process of data entry will usually involve a data manager who designs the data entry screens, while other staff, such as data entry clerks or field staff, do the actual entering of the data. Creating data entry screens is not difficult but requires care. The data entry screen should follow the questionnaire, so automatic skips can be used to follow the skips in the questionnaire, and drop down menus can be used to show the same options as in the questionnaire for individual questions.
If data are being entered from paper-based forms, data entry can be double or single. Double data entry is routinely used to minimize typing errors and to ensure that the data in the database accurately reflect what was recorded on the forms. There are two main techniques used for double data entry. In one method, two data entry clerks independently enter the data without any knowledge of the other’s work, and both entries are stored. A program, which may have to be written by the data manager, compares the two entries and identifies any discrepancies. The resolution of the discrepancies is generally referred to as ‘verification’ and can be done in different ways. In some systems, verification may involve both data entry clerks re-entering the specific data fields where discrepancies were identified and comparing the new entries. More commonly, a third person resolves the discrepancies, by referring to the original forms or questionnaires, and makes a decision as to the correct entry, changing the incorrect entry appropriately. Once the data have been double-entered and no discrepancies between the two entries are identified, the data are considered verified.
In the other method, which is used in specialist data entry programs, such as CSPro, the second person resolves the mismatches at the time of entry. After the first entry is complete, a second person enters the data, and any discrepancies are flagged up immediately, as the data are being entered. The second person must decide what the correct value should be and enter it accordingly. With this method, the second person is generally chosen to be more experienced and is expected to make the final entry, corresponding to what is on the form. This method is quicker but more prone to error than the first method described.
Facilities for double data entry are a feature in some software packages, for example, Epi-Info. In other packages, such as Access, double data entry must be set up when the database is created and would require considerable time and skill. One option is to use a package, such as Epi-Info, for data entry, before transferring the data to a separate package, such as Access, for storage and management.
Single entry of data is relatively rare and not recommended. It should only be considered if there are extensive checking routines, strong supporting processes, and technology in place to identify possible errors. Generally, the cost of doing a second entry of data is less than the costs of the additional data management required to clean the errors that that may remain after just a single entry of data.
The task of entering data should be conceptually separated from the task of analysis. Different software may be used for data entry, data checking, and analysis. The data entry system should be designed to make data entry as simple as possible. Simplifying the keying process will speed the task and make it less error-prone. Ideally, the data entry screens should closely resemble the paper form from which the data are being copied. Questionnaires should also be designed with data entry in mind. The data should be entered as recorded on the questionnaire. No hand calculations or transformation should be done before data entry—these can all be done during the analysis stage.
5.2 Data checks
Most data management time is taken up with checking the study data for errors and inconsistencies and ‘cleaning’ it. There are three main points to be considered when developing data checks: (1) deciding what will be checked; (2) working out when each check will be used; and (3) specifying how to resolve inconsistencies and errors identified by the checks.
Many software packages have inbuilt facilities for data checking. These automatic check programs can be set up when the database is created and run at different stages of the data management process. Before the check program is created, a specification document should be prepared, defining the data that will be checked and the errors that the program will be designed to catch. This document is usually written by the data man- ager or statistician, with input from the investigators, and is known as the data validation plan or check specification plan. Generally, the person who sets up the study database will also write the checking program and will be responsible for testing it. The program should be tested on ‘dummy’ data, before using it on the actual study data, to ensure that it is working correctly. Several ‘dummy’ questionnaires can be completed with deliberate errors and entered into the database; these can be used as test data for checking purposes.
Data checks can be incorporated into the data entry screens, so that illogical or implausible values are flagged up at the time of entry. Furthermore, the entry screens can be designed, so that they do not allow entry of invalid values, such as an impossible date or a value of ‘5’ for a question that has ‘1’ to ‘4’ as the only possible answers. There are arguments for and against using data checks at the time of data entry. The checks will slow down the entry process, and, with double data entry, it can be argued that the checks are not needed to pick up errors in data entry, but they will pick up some data that have been incorrectly recorded on the questionnaire. If checks are incorporated into the entry screens, they should be designed so as to allow entry of invalid values, if that is what is recorded on the questionnaire. Otherwise, the questionnaire must be put to one side, until the error is resolved, and risks being lost or misplaced, so that it never gets entered in the database. Except for very small studies, it is often better to get the data entered in the database and to run checks to identify any errors afterwards, especially where there are a large number of questions that might need checking. However, interactive checking may be preferable when data are entered by the same field staff who completed the questionnaires earlier in the day. They may be slow typists but, having the interviews fresh in their minds, are more likely to be able to correct errors at the time of entry. However, in this case, it would probably be even better to consider having the data entered at the time of the interview, using electronic data capture methods.
After data entry is complete and the data are verified, an automatic check program is run to identify errors. These checks can include range checks to identify out-of-range or missing values (for example, dates out of the expected range, participant’s age outside the range permitted by the study protocol) and cross-checks to identify inconsistencies between values (for example, males who are pregnant). The timing of when these checks are run requires careful consideration—for example, if a check compares data from different visits, the data from both visits must be present for the results to be meaningful.
In a longitudinal study, with repeated data collection visits to each subject, data checks should be run early on and continuously throughout the study. When errors are identified early in the study, it is often possible to uncover misunderstandings in the interpretation of the questionnaire or a flaw in the questionnaire design that was not picked up during the pilot phase. Clarification or further training can prevent those problems from recurring throughout the entire study.
The initial analyses are a continuation of the checking process and should include looking at cross-tabulations of the data to identify inconsistencies, and scatter plots and box-plots to compare groups and identify outlying observations. In large longitudinal trials, interim tabulations of data are recommended as a way of detecting possible data errors. Special checks might be made on observations that are more than two or three
standard deviations from the mean. Such observations should be checked individually, as they are not impossible, merely unlikely.
Lastly, it should be noted that discrepancies in data are time-consuming to identify and resolve. The implications for data checks and query resolution should be considered during the questionnaire design stage. Asking for duplicate information in different parts of the questionnaire is one source of unnecessary queries. Queries will also arise if the questionnaire design does not make adequate provision for unavailable responses or permits ambiguous responses. Questionnaires frequently include questions that are deliberately used as cross-checks of other fields. This can be a very good policy when the data are actually different. For example, a check of sex against the subject’s pregnancy status provides a reasonable cross-check of whether the person could not possibly be pregnant because they are male. However, problems arise when questions duplicate the same data, for example, a questionnaire that asks to record both the age and birth date. Discrepancies and confusion are bound to be generated when the values do not agree. When designing a questionnaire with requests for repeat information, consideration should be given to the implications for data checking and whether the duplicate information is truly needed.
5.3 Data cleaning
Data cleaning involves raising and resolving data queries that are identified during the data checking process and making the appropriate changes to the database to correct the errors. The aim is to be sure that the data are as of high quality as possible, before they are analysed.
Some data queries can be resolved within the data management group, for example, an obvious error in the year of a visit date. However, most problems will need to be resolved by the field team or the investigator. Commonly, the data management group will send a list of queries to the field team; the team will resolve the queries by writing the correct answer next to each one and return the list to the data management group. Alternatively, corrections can be made to the questionnaire itself. However, it is very important that the original answer is not obscured—instead, it should be crossed through with a single stroke, so that it is still legible, and the new information written on the side. It is good practice (and required for GCP) for the person making the change to initial and date the changes.
After the queries have been resolved, the database should be updated to reflect the corrected information. Some software systems have features to allow changes to the data after query resolution. These changes are usually made through the data entry screens and are recorded in an electronic audit trail. In a system without an automatic audit trail, changes may be made directly to the data tables themselves, although this can be more error-prone than changing the data via the data entry screens.
Correction or editing of data to reflect a resolution generally follows a different path from that of initial entry of the data. Most systems do not support double entry of corrections, so it is good practice to have a visual check of the data after correction to be sure that the change was made correctly. After the changes are made, it is essential to re- run the check program again, since it is possible that the update of the data has caused a new inconsistency to be identified.
In some cases, it may not be possible to obtain a resolution to a query, particularly if it is some time since the data were collected. Some software systems keep an electronic record of the problems identified by the check program, and a code can be entered to indicate that the inconsistency cannot be resolved. Alternatively, the incorrect data can be given a code for ‘missing’ if the correct answer cannot be obtained. The number of times this is done should be kept as small as possible and should be documented.
It is important to have a single master copy of the database that contains all the data corrections that are made. Even after the data cleaning stage is complete, errors may be detected much later during analysis; these should all be corrected on the master copy, so that it is always up to date. A version control system should be in place to ensure that it is possible to know which version of the database was used for any particular analysis.
5.4 Variable naming and coding
One of the first things to be done, before developing the study database, is to create an annotated questionnaire, containing the names that will be given to different variables and the characteristics of the variables such as numeric or text, the length of the variable (maximum number of characters), and any specific code lists that will be used. Ideally, variable names should provide information about the data being recorded, for example, ‘birthwt’ for birth weight or ‘intdate’ for interview date. In a longitudinal study, the same name should be used for those variables that are recorded at every visit. Some studies have used a convention of naming the variables at different visits with a number at the end corresponding with the visit, for example, ‘visit_date3’ and ‘visit_date6’ for visits 3 and 6. However, it is generally easier to run data checks and do other manipulations on the data if the variables have exactly the same name at each visit. An additional variable for the visit number should be included to identify the visit.
Some software packages have restrictions on the length of variable names; in particular, some older packages do not allow more than eight characters. A good general rule is to use no more characters than are allowed by the most restrictive (in terms of the number of characters) software package that is likely to be used for the study.
Questions that have categories of answers are best entered as coded values, rather than text (for example, 0 = No, 1 = Yes). These fields have a limited list of possible answers and only present a problem for data management if the field can contain more than one answer or if the answer falls outside the predefined list. When more than one answer is possible (for example, a list of types of contraception ever used), the database design changes from a single field to a series of fields, each of which can hold any of the valid responses from the list (coded yes or no). If an answer occurs that is not on the predefined list, a value for ‘other’ may be needed. In this case, it is advisable to create an additional database field where the specific response can also be entered as text. If an answer that is not on the list occurs frequently, it may be worth creating a new code for it. New codes may also be needed if the questionnaire design changes during the study or between survey rounds. In these cases, it is essential that the existing code list does not change. Instead, the new codes should be added at the end of the list. The coding of responses to questions is dealt with in more detail in Chapter 14.
Some studies use free-text on the questionnaire and re-code the text into categories at the time of data management. Re-coding variables in this way is generally not recommended, as, if they have been collected and entered, the original data should be found in the final data set. If any re-coding is done after data entry, the new data should be put into a new variable, with a note to indicate how the variable and the codes were defined.
5.5 Data lock
When all the data checks have been run, the queries resolved, and all QC activities are complete, the data are declared ‘clean’, and the database is ‘locked’. This means that no further corrections will be made to the data. In GCP-compliant studies, the trial cannot be unblinded, i.e. the randomization codes made available to the study team and the main analyses cannot be performed, until the database is locked. At this stage, the locked database may be deposited with an independent body, such as the data safety and monitoring committee (DSMC), so that, if there are any later queries about the integrity of the data or changes that may have been made, comparison can be made with the locked set.
Even in studies that are not being run to be fully GCP-compliant, it is useful, at some stage, to make the formal decision that the data are ‘closed’, and no more corrections will be made. Sometimes, the data can be closed for one analysis, while corrections are ongoing for other data. The purpose of ‘closing’ the database is to ensure that the data are defined for a stable set of analyses and will not change every time the analysis program is re-run. The closing of the database, or any part of the database, should not be done before all the errors that are correctable have been resolved.