As stated earlier, accessibility in this case is simply
the average time between a census tract centroid and all
26 hospitals. Since the time by Google is systematically longer than the time by ArcGIS, the comparison based on
the time itself is less meaningful. Accessibility is to measure
relative ease of reaching an activity or opportunity.
Therefore, each series of average travel time to hospitals
needs to be standardized (with a zero mean and standard
deviation of 1) to be comparable. The resulting (standardized)
Z value reflects the relative accessibility. A higher Z
value corresponds to more travel time to hospitals and thus
a poorer accessibility.