This is a test of the camera's noise levels. The higher the signal to noise ratio, the greater the difference in strength between the real image data and random background noise, so the 'cleaner' the image will look. The higher the signal to noise ratio, the better.
Pentax K-S2 signal to noise ratio
JPEG analysis: The K-S2 delivers similar results to the E-M5 II and EOS 700D right up to ISO 1600 and opens up a slight gap at higher ISOs. It's clearly better than the Nikon D5500, even though the K-S2 also dispenses with the anti-aliasing effect.
Raw analysis: Here, the Pentax fares slightly less well compared to its rivals. The innate noise levels in its raw files appear to be slightly higher, though it still beats the Nikon D5500.