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 Canon EOS 70D match the marked ISOs within 1/6 stop accuracy, meaning ISO 100 indicated = ISO 100 measured.
Noise and Noise Reduction (JPEG)
At its default ('Standard') level of noise reduction, the EOS 70D exhibits similar image noise to many of its peers between ISOs 1600 and 6400, and somewhat less than its predecessor. Noise suppression increases somewhat at the camera's two highest ISO sensitivities. And as the samples bear out, image detail falls off significantly beyond ISO 3200.
Although the numbers show the 70D offering slightly lower noise levels than the Nikon D7100 at ISO 12,800 and 25,600, the difference in the samples isn't significant. Unlike the 60D, the 70D offers a one-stop-higher ISO 25,600 setting.
ACR noise (ACR 8.2, noise reduction set to zero)
As the sample images reveal, noise from the 70D's Raw files is smoother and less blotchy than its predecessor from 1600 to 6400. The files aren't quite as smooth as the Nikon D7100, and they're no match for the very smooth X-Pro1 (which may well be doing some noise reduction as part of the processing applied to its Raw files), but there is some improvement over the 60D.
Looking at the graph of Raw results, you can see the 70D is slightly better than the 60D, but not by a wide margin. Neither is it significantly different to the results from the D7100.