Data Issues

Waves 1 to 7
Data issues - Waves 1 to 7 – February 2019

18 Study children allergies (issues with Wave 6 and 7 data)

In Waves 5, 6 and 7 the P1s in B and K cohorts were asked about the study child's existing allergies to food items and also asked if there had been any reactions to food items since the previous interview. The relevant questions are HEAL 17 and 18 with the word sub 'allergy' sourced from roll forward (pre-fill) data:

  • HEAL_Q17* (hs43*) - In a previous interview, you told us that [(SC)] had a reaction to [(allergy)].
  • HEAL_Q17* (hs43*) - Has [(SC)] had any more reaction to [(allergy)] since we last saw you?
  • HEAL_Q18* (hs39*) - Since the last interview, has [SC] had a reaction (e.g. redness or itching) that you thought was due to some food or drink [SC] had consumed?

While processing the Wave 7 data some issues were discovered that originated in errors made in the roll forward (pre-fill) process. These errors have affected the Waves 6 and 7 data for some records in B and K cohorts. For some records, the historical allergy information was not rolled forward correctly resulting in the participant not being asked when they should have been. There were also output errors in the release data for Wave 6. The following section contains detailed explanations of the issues.

18.1 Wave 6 data issues

Roll forward from Wave 5 to 6 (Pre-existing allergies HEAL 17)

For Other Nuts, Milk, Wheat and Other Allergies there were data roll forward errors. If a respondent answered 2 (Withdrawn food) or 3 (Still have allergy) in Wave 5, the response should have been rolled forward to pre-fill the Wave 6 instrument. This wasn't done due to processing errors that occurred while producing the pre-fill files. This meant that for these allergies Q17* was not asked and therefore data on whether the study child continued to experience these allergies is absent from Wave 6. The number of records affected are shown in Table 33.

Table 33: Wave 6 records affected by pre-fill errors
  Other nuts Milk Wheat Other allergies
B cohort (records affected 28 39 14 170
K cohort (records affected 27 25 13 158

For all other allergy types (Peanut, Eggs, Soy, Sesame) the roll forward of the pre-fill information was correctly done.

Roll forward from Wave 5 to 6 for new reactions (HEAL 18)

All correct.

Error in output data for Wave 6 - HEAL 17

There was an error in Wave 6 output data for the responses to questions about existing allergies (HEAL 17). The addition of a number of other allergy categories or collection caused a mapping error from the input data through to the output data. The input data is correct for all allergies (not affected by the roll forward data problem) but the input data for the following allergy types were mapped to the incorrect output item:

  • Fruit
  • Preservative
  • Additive
  • Seafood
  • Other allergies.

The number of records affected are shown in Table 34.

Table 34: Records affected by output mapping errors
  Fruit Preservative Additive Seafood Other allergies Total
B cohort (records affected) 24 27 34 9 13 107
K cohort (records affected) 15 10 16 11 19 71

These have been corrected for the Wave 6 data as part of the Wave 7 data release.

A flow-on from this error was that the incorrect version of the output data was used as the basis of the pre-fill information for the Wave 7 allergy information, so it has also had an effect on the quality of the Wave 7 data (see Wave 7 data section below).

18.2 Wave 7 data issues

Roll forward from Wave 6 to 7 (Pre-existing allergies HEAL 17)

The creation of the roll forward file for Wave 7 relies on the use of the output data to ensure the most current data are used. As mentioned above, the output file for Wave 6 was incorrect (due to the output mapping issue). So, all of the roll forward indictors for Fruit, Preservative, Additive, Seafood and Other allergies were incorrect - 107 records in B cohort and 71 records in K cohort were affected as a result of no indicator data being rolled forward for any of these allergy types (as per Table 34 above.)

There was also another separate processing issue with the roll forward of Milk where some participants in Wave 7 were asked about existing milk allergies incorrectly; that is, the study child did not have an existing allergy to milk but was asked Q17* about whether they had experienced any further reactions in error at interview (24 records in B cohort and 6 records in K cohort). This error occurred due to a processing data extraction issue, not as a result of the incorrect mapping problem described above.

For all other allergy types (Peanut, Other nuts, Eggs, Soy, Sesame and Wheat) the roll forward was correctly done between Waves 6 and 7. For records where roll forward was done incorrectly between Wave 5 and Wave 6 for Other Nuts, Milk, Wheat and Other allergies, the error has carried through to Wave 7 data; for example, those records were still not asked about their existing allergy from Wave 5.

Roll forward from Wave 6 to 7 for new reactions (HEAL 18)

All correct.

18.3 Corrections made

In relation to the mapping issue, Wave 6 output data have been fixed so that the input data are correctly reflected in the output items in the released data files.

In relation to the Wave 5 to Wave 6 roll forward issue, any data items for records that were in scope to be asked about known pre-existing allergies (including the subsequent reaction type variables), but were not asked, were set to missing.

Any data items (including the subsequent reaction type variables) for records that were asked about an existing milk allergy in error were set to -9 (not asked).