Stratification of the Population and Weighting of the Results
Earnings and expenses estimates are published by a range of characteristics including dental type, contract type, gender and age band. These figures are based on a sample and are weighted according to the entire self-employed NHS dental population for the country and dentist type in question.
Results by weekly hours and percentage of dental time spent on NHS/Health Service dentistry (derived from the survey data) are based on a survey sample and are weighted according to the self-employed primary care NHS dental population.
Results by business arrangement (also derived from the survey data) are based on a sub-set (those Providing-Performers/Principal dentists that answered the business arrangement question) but are weighted according to the self-employed Providing-Performer/Principal population as applicable. These results are now no longer available in the main report but can be found in the time series and CSV,
As the results throughout the report are estimates based upon samples that have been weighted to the full self-employed NHS dental populations, they are subject to sampling error. Differences between groups and sub-groups of dentists or between reporting years may not be statistically significant and where significance has been tested this is noted in the report.
To weight the population as needed, dentists are allocated to strata according to their characteristics.
Stratification – England and Wales
Until 2017/18, figures for England and Wales were presented as a combined average, and the self-employed NHS dental population in England and Wales were allocated to one of 32 strata according to:
- dental type (Providing-Performer or Associate);
- contract type (GDS, PDS or Mixed GDS/PDS);
- gender (male or female);
- and age (under 35, 35 to 44, 45 to 54 or 55 and over).
- Full stratification of all four variables would lead to a possible of 48 strata, but the low numbers of mixed contract dentists do not allow for this cohort to be split by gender or age.
- As the PDS population is small and reducing, the two age bands of <35 and 35-44 years for Providing-Performer and Associate dentists were combined.
However, since last year separate figures for England and Wales have been made available and the combined averages are no longer available. The changes have also led to a review, and revision, of the stratification and weighting methodologies used in England and Wales.
Due to a continued decline in the number of PDS or mixed (GDS/PDS) contract holders, contract type is no longer included in the strata for England.
This means that the dental population in England is now allocated to one of 16 strata according to:
- dental type (Providing-Performer or Associate);
- gender (male or female);
- and age (under 35, 35 to 44, 45 to 54 or 55 and over).
Due to a much smaller dental population in Wales, it is not possible to stratify dentists using the same breakdowns as used in England. Instead, the strata in Wales now matches that of Northern Ireland and Scotland and six strata are used to weight the figures:
- gender (male or female);
- and age (under 35, 35 to 44 or 45 and over).
This is consistent with the stratification methodology used within the Dentists' Working Patterns, Motivation and Morale - 2018/19 and 2019/20 report. These changes, alongside the change in methodology mentioned above, mean that the England and Wales figures published in 2018/19 and later are not comparable to those of earlier publications and should not be compared with earlier figures. Separate England and Wales figures for 2017/18 have been recalculated using the new methodology. Please refer to the methodological change notice for further details.
Due to the low population of dentists who perform more orthodontic work than non-orthodontic work (units of orthodontic activity > units of dental activity), a different system of weighting was employed and the age variable was replaced by a variable flagging the dentists who primarily undertake majority of orthodontic work and those who do not. These results can be found in the Excel time series and the weighted average taxable income estimates for all self-employed primary care dentists in that section differ slightly from the averages shown in the rest of the report.
The Expenses to Earnings Ratio (EER) figures presented in report tables are calculated by dividing average expenses by average gross income for the applicable grouping which is not the same as the weighted average EER for that group.
Stratification - Northern Ireland, Scotland
Because of the smaller sample sizes in Northern Ireland and Scotland, the self-employed primary care NHS/Health Service dental population is allocated to one of six strata according to:
- gender (male or female);
- age (under 35, 35 to 44, 45 and over).
Due to the low population of dentists who were classified as ‘Orthodontic Items/Courses ≥ 30%’, a different system of weighting was employed for the analyses presented in the applicable sections of the report. In these cases, a variable for dentists classified as ‘Orthodontic Items/Courses ≥ 30%’ and ‘Orthodontic Items/Courses < 30%’ was added as an additional stratum to the variables described above (thus resulting in 12 strata). This means that the weighted average taxable income estimates for all self-employed GDS dentists in these sections differ slightly from the averages shown in the rest of the report.
In Northern Ireland only, for analyses by percentage of dental time spent on Health Service dentistry, the groupings used to analyse this variable (≤25%, >25<75%, ≥75%, and no survey response) are added as an additional stratum resulting in 24 strata. This means that the weighted average taxable income estimates for all self-employed GDS dentists for these analyses will differ slightly to the average shown in the rest of the report.
Calculating the Percentage of Earnings from Health Service Dentistry in Northern Ireland
To derive the measure of “Percentage of Gross Earnings from Health Service Dentistry” in Northern Ireland gross earnings from Health Service dentistry (supplied by BSO) is divided by total gross earnings provided by HMRC to produce a percentage of a dentists’ total gross earnings from Health Service work. This estimated measure but is subject to caveats:
- BSO reports Health Service earnings for Associate as well as Principal dentists. However, in some cases, part of the Associates’ payment for this work will be given to the contract holder. Thus there will be some cases where BSO’s reported Health Service gross earnings will be higher than the total gross earnings reported by the Associate on their Self Assessment tax form. This is because total gross earnings will be net of the payment made to the Principal in whose practice they work.
- Similarly, the corresponding Principals’ Health Service earnings may be subject to some under-counting.
Principal dentists in Northern Ireland may also receive income from the Heath Service for providing education to dentists in training (Assistants, Foundation Dentists/VDPs and GPTs). Where the Self Assessment tax return has been completed correctly, this income will not be declared as gross earnings, but used to ‘offset’ the expenses incurred for providing such services. However, this income is included as earnings in the BSO Health Service figures which means that the percentage of earnings from Health Service activity will be overstated for many Principal dentists providing such education services.
It is not possible to quantify the extent to which the issues described in this section affect the results of the analyses, but to some extent the effects will be negated for results shown for all dentists, and will only affect those dentists near the boundaries of the three Health Service percentage bands (>0≤25 per cent, >25 to <75 per cent, ≥75 per cent).
This measure is fundamentally different to the measure of percentage of time spent on Health Service dentistry used in the Dentists' Working Patterns, Motivation and Morale report, and the two cannot be compared.