I have clinical evaluation for patients every six months. This evaluation relies on a scale between (0-48). Maximum is better/lower is worse. I have numeric value of this scale every six months. I want to measure the rate of change in this scale every six months. Which one of these methods is correct:
Method#1:
Rate of change = (max score - current score)/ six months
Method#2:
Rate of change = (previous score- current score)/ six months
Second option is the one you should go for, it tells you the change in score per month
$$ \text{Rate of change} = \frac{\text{Previous score} - \text{Current score}}{6~\text{months}} $$