This disclosure relates to characterising data sets that are distributed as multiple data subsets over multiple computers such as by determining a gradient of an objective function. A computer determines a partial gradient of the objective function over a data subset stored on the computer and determines random data. The computer then determines an altered gradient by modifying the partial gradient based on the random data and encrypts the altered gradient such that one or more operations on the altered gradient can be performed based on the encrypted gradient and sends the encrypted gradient. Since the partial gradient is altered based on random data and encrypted it is difficult for another computer to calculate the data that is stored on the first computer. This is an advantage as it allows to preserve the privacy of the data stored on the first computer while still allowing to characterise the data set.