OHI+ Tanzania_Technical Report_2018

Ocean Health Index – Tanzania 2018 39
8.6. Method - calculating the index
The index (I global ) score is the sum of the regional
scores (I region ). Regions are combined by areaweighted
average (a).
The regional scores (I region ) are the sum of all the
individual goal scores (G i ). All goals were given
equal weight in the index (w g ).
The goal score (G i ) is the average of current status (50% of the goal score) and probable future state (50%
of the goal score).
The goal current status (x) is calculated using available indicators in relation to a stakeholder set reference
point (see goal models & data below for specific models for each goal). The probable future status (x f )
measures the cumulative pressures (negative impacts on a goal score), cumulative resilience (positive
impacts on a goal score) and trend (projected goal status in five years). The future status score is the near
future score projection of a goal calculated as the current goal status multiplied by the 5 year status trend
(T) and the difference in the resilience (r) and pressure (p) applied to that goal (Halpern et al., 2012).
The trend is given a higher weight (β) than the pressure/resilience component. This value was chosen
based on the assumption that trend is a better indicator of the near future condition than indirect
measures of pressure and resilience (Halpern et al., 2012). All pressure and resilience values are scaled
from 0 to 1 and trend is constrained to -1.0 to +1.0.
Pressures and Resilience
Cumulative pressures acting on each goal are calculated as the sum of ecological (P E ) and social pressures
(P S ):
Where γ is the relative weight for ecological vs. social pressures, set as equal (0.5). Each pressure layer is
assigned to an ecological or social category, with ecological pressures further assigned to one of five
subcategories (pollution, alien species, habitat destruction, fishing pressure, and climate change). Each