We address the problem of choosing synchronous data combinations from periodically generated data sequences, or pgds for short. In real time systems, pgds play a major role. In integrating pgds's generated at various information sources, we face the problem of choosing which data combinations to use, since different pgds, in general, have different data generation intervals. A data combination is defined as a set of data each obtained from distinct pgds. As a criterion for choosing a data combination, we propose a novel notion of synchronousness of data combinations. We present two theorems for calculating upper bounds of the synchronousness of data combinations, given a set of pgds: one for pgds with the same periodic interval and the other for pgds with different periodic intervals. These theorems give the synchronousness upper bounds that guarantee to find the data combinations at the constant interval that is the same as the maximum periodic interval of all the pgds.