The actual sensitivity of each indicated ISO is measured using the same shots as are used to measure ISO noise levels, we simply compare the exposure for each shot to the metered light level (using a calibrated Sekonic L-358), middle gray matched. We estimate the accuracy of these results to be +/- 1/6 EV (the margin of error given in the ISO specifications). In our tests we found that measured ISOs from the Sony DSC-RX100 match the marked ISOs within 1/6 stop accuracy, meaning ISO 125 indicated = ISO 125 measured.
Noise and Noise Reduction (JPEG)
The RX100 does a good job of balancing noise and noise reduction up as far as ISO 1600. Beyond this point it increases noise reduction drastically and detail suffers as a result. Overall, though, it's a very good performance for such a small camera - it's noticeably only the G1 X and Nikon 1 that offer comparable performances (though cameras such as the Olympus XZ-1, Samsung's EX models and the Fujifilm X10 offer brighter zooms, so can keep to lower ISOs for longer).
RAW noise (ACR 7.1, noise reduction set to zero)
As usual, looking at the Adobe Camera Raw results gives a pretty clear explanation of the JPEG results - noise is pretty well controlled until ISO 800 and then ramps up significantly with each ISO step increase from there. At ISO 1600 there's enough noise to explain the loss of detail that begins in the JPEGs, but at ISO 3200 and 6400 it's clear why the JPEGs become so smudged - the noise levels will have left Sony with little other option.