Another example:

• • Another example:
  • • Subject 1 - signal in hippocampus has a mean of 1000, and goes from a baseline of 990 to a response at 1040
    • Difference = 50 MRI units
    • Subject 2 - signal in hippocampus has a mean of 500, and goes from a baseline of 500 to a response at 525
    • Difference = 25 MRI units
  • Conclusion: each shows a 5% change, relative to the mean.
  • • these changes are 5% above the baseline
    • But 5% of what? It is 5% of the mean.
  • Percent of baseline might be a slightly preferable scale (to percent of mean), but it may not be worth the price.
  • • the difference is only a fraction of the result
    • • e.g. a 5% change from the mean would be approximately a 5.1% change from the baseline, if the mean is 2% above the baseline
    • computing the baseline accurately is confounded by using motion parameters (but using motion parameters may be considered more important)