The graph on the right shows two density functions (also called histograms) of the UCTE grid frequency:

Ideally, the histogram forms a normally distributed bell curve, i.e. both sides are symmetrical. However, the real frequency distribution often deviates from the ideal. For example, the histogram of the network frequency can be slightly left-shifted (network time slower than world time) or right-shifted (network time faster than world time). The grid time is measured by the Swiss transmission grid operator Swissgrid. If there are deviations in the grid time of more than +/- 20 s compared to the world time, a correction must be made. These time corrections are made by adjusting the grid frequency. This can lead to shifts in the frequency histogram compared to the previous day.