What is the sample rate of my ATmega88's ADC?
The sample rate of most ADCs is completely independent from it's bit resolution.

The number of bits returned by an ADC is a function of it's hardware.
Basically, a 10 bit ADC is a 10 bit ADC. If you're not using some of the bits, it's still a 10 bit ADC.



As such, the maximum achievable sample rate with the ADC in the ATmega88 at the full 10-bit resolution is 15 kSPS. However, you can run the ADC faster, at the expense of increased noise.

From the ATmega88 docs:


  By default, the successive approximation circuitry requires an input clock frequency between
  50kHz and 200kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the
  input clock frequency to the ADC can be higher than 200kHz to get a higher sample rate.


So basically, as the speed at which you clock the ADC increases, the ADC noise increases. As such, if you run the ADC faster then the 15 kSPS, you still get 10 bit conversion results, however, the smaller bits are not valid, as they are swamped by noise and/or biases in the ADC.