Egenvall et al. Acta Veterinaria Scandinavica 2011, 53(Suppl 1):S4 http://www.actavetscand.com/content/53/S1/S4 with more than 50 registered tumours of all kinds during the study period [3]. It is noted that the method is uncertain because the knowledge about the popula- tion size is approximate only. The geographical variation in the incidence rate of canine atopic dermatitis (CAD) in Sweden has been investigated and the association between such variation and selected environmental risk factors evaluated [21]. The unit of analysis was the postal code area (PCA), and information on the incidence rate of CAD from the Agria insurance company was aggregated to this level based on the postal code of the owner’s address. The database (n=220, 835 insured individuals with 1,245 with at least one claim for CAD, out of which 1,235 were matched to one of the 559 postal code areas) was merged to a digitized map of Sweden which contained the spatial location of the PCAs as well as the human population density per PCA. Additional meteorological information (average temperature, rainfall and more) was added from the Swedish National Atlas [22]. Infor- mation from the three databases was combined and used to visualize and explore spatial relationships in the incidence of CAD, as well as to generate a “risk map” based on the predictions from a generalized linear mixed Poisson-regression model with PCA as random variable [23]. Merging at the individual level Data from three sources; pedigree data [24], young horse quality contest data and competition results [25] were integrated [7]. There were 3,708 Warmblood horses (born between 1968 and 1982) that had partici- pated in the young horse quality contests as 4-year-olds and 25,605 horses (born between 1953 and 1995) with competition records, all of which had an identified pedi- gree. For dressage and show-jumping 1,206 and 1,879 horses were available for analysis. Any problems found during merging were not mentioned, likely because both the competition database and the RHQT rely heavily on correct pedigree recording, making merging straight for- ward. In addition, the young horse quality contests were entered manually from written protocols. One problem/ benefit discussed regarding the ability to extrapolate the results was that selection bias was considered small because 35% of 4-yr old horses participated in the young horse quality contest [7]. From an epidemiologi- cal point of view this proportion can both be considered large or small; large as to actually have attracted many horse owners but too small to have convincingly covered a majority of the population. Information on five dog breeds from insurance and kennel club data (n>28,000 dogs) was integrated to study hip dysplasia (HD) [12]. The details of the integra- tion was as follows: by breed “between 61% (German Shepherds) and 77% (Rottweilers) of all dogs registered Page 3 of 6 by the Swedish Kennel club with a Swedish registration number born during 1994–2003 had an official screen- ing result for HD”. Among the breeds analysed, 58% of the dogs from the insurance database were included. This can be considered acceptable because not all dogs are screened for HD. Also, “the proportion of dogs with a hip screening result in the SKC data that also had a life and/or veterinary insurance in the edited data from Agria ranged from 36% to 51%”. It was argued that it should be possible to generalise the results to a larger dog population, at least for these breeds. Data identifiers could have been better than they were in the insurance data but checking of the data showed correct merging (results not presented). Data from one large regional animal hospital and pedi- gree data were studied relative to osteochondrosis lesions [26]. The data used included information on horses screened (prior to breaking, sale or breeding evaluation; n= 879) for osteochondrosis and other osseus fragments as well as horses examined due to clinical symptoms from the locomotor apparatus (n=3,639). The estimated heritabilities were within the range of results published previously. One reason this study was thought to have good results was that the same radiologist examined all the radiographs, which may of course also lead to bias if this radiologist differed in his/her evaluations from the general consensus. However, 26% of the horses were lost because of lack of valid animal identity- which can be considered a substantial fraction. It was argued that providing routine clinical data for research could be valu- able. However, the animals analysed must be an unbiased sample of the population from the analytical aspect. This is most likely to happen when one clinic will be the sole provider of a specific service or if data from several clinics can be combined. This may be a situation more often found in the Nordic countries because of the relatively long distances between clinics. We advise that clinical data not emanating from screening programs should be used even more cautiously. Discussion What is needed for individual or group-level merging? A pre-requisite for integration is that the individual identifier(s) must be correct and linkable across data- bases, which might explain why such studies are still rare. Ideally, merging should be done on a unique indi- cator for each animal, e.g. registration number or chip number. “Second best” is if, using predefined categories (e.g. breed codes), the merge may be performed using several identifiers. For example, if merging two data- bases with pedigrees where imperfect identity is found in one of them, the merging by name, breed, year of birth and grandsire is more accurate than merging only on name, although more records may be lost. It may be