  We present an overview and statistical analysis of the data included in
WEBDA. This database includes valuable information such as coordinates,
rectangular positions, proper motions, photometric as well as spectroscopic
data, radial and rotational velocities for objects of open clusters in our
Milky Way. It also contains miscellaneous types of data like membership
probabilities, orbital elements of spectroscopic binaries and periods of
variability for different kinds of variable stars. Our final goal is to derive
astrophysical parameters (reddening, distance and age) of open clusters based
on the major photometric system which will be presented in a follow-up paper.
For this purpose we have chosen the Johnson UBV, Cousins VRI and Stroemgren
uvbybeta photometric systems for a statistical analysis of published data sets
included in WEBDA. Our final list contains photographic, photoelectric and CCD
data for 469820 objects in 573 open clusters. We have checked the internal
(data sets within one photometric system and the same detector technique) and
external (different detector technique) accuracy and conclude that more than
97% of all investigated data exhibit a sufficient accuracy for our analysis.
The way of weighting and averaging the data is described. In addition, we have
compiled a list of deviating measurements which is available to the community
through WEBDA.
