How the Isolation Index works
The Isolation Index is a statistical model we use to understand the relative isolation of schools and kura.
The Isolation Index formula
The Isolation Index is calculated for schools on the North and South Islands using the below formula:
(0.8 x d5) + d20 +(0.4 x d60) + (0.8 x t5) + t20 + (0.4 x t60) ÷ 200 = Isolation Index
d5 = distance in km to population centre of at least 5,000 people
t5 = driving time in minutes to population centre of at least 5,000 people
d20 = distance in km to population centre of at least 20,000 people
t20 = driving time in minutes to population centre of at least 20,000 people
d60 = distance in km to population centre of at least 60,000 people
t60 = driving time in minutes to population centre of at least 60,000 people
- Where the nearest population centre to a school (or the population centre it is located in) has 60,000 inhabitants it is counted as the 5,000, 20,000 and 60,000 population centres.
- If the nearest population centre has 20,000 inhabitants it counts as both the 5,000 and the 20,000 population centres.
- A school could be measured against up to three different population centres.
Schools on offshore islands
When the Isolation Index was initially introduced in 2002 schools and kura on offshore islands were given a notional Isolation Index number.
We do not believe that there have been significant changes in the relative isolation levels of schools and kura on offshore islands over the last 20 years. We have however updated the notional values each of these schools have to reflect the changed scale of the Isolation Index.
We calculated this by looking at the differences between the old and new Isolation Index values for similarly isolated mainland schools. We then applied the difference for offshore islands.
How are distance and travel time calculated?
Distance and time have been calculated by an independent specialist company.
The road data is sourced from local authorities across New Zealand and also government agencies including NZTA.
Distance is measured on the fastest route over navigable public roads. The fastest route means that roads like motorways or state highways will be prioritised over bus routes, roads with lower average speed, and inappropriate routes, such as back roads.
Time reflects an average of the actual speed when driving between locations using the same roads as the distance calculation. It does not take into account time of day differences, or day to day changes in road conditions.
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