Editing Price index (section)

=== Calculating indices from expenditure data ===
Sometimes, especially for aggregate data, expenditure data (<math>E_{c,t} = p_{c,t} \cdot q_{c,t}</math>) are more readily available than quantity data. If expenditure and base prices are known, indices can be computed without direct quantities. For example, Laspeyres becomes:<ref>Statistics New Zealand; ''Glossary of Common Terms'', [http://www2.stats.govt.nz/domino/external/omni/omni.nsf/0/7d11773239ed84eecc256f350016b841?OpenDocument "Laspeyres Index"] {{Webarchive|url=https://web.archive.org/web/20120206210828/http://www2.stats.govt.nz/domino/external/omni/omni.nsf/0/7d11773239ed84eecc256f350016b841?OpenDocument|date=2012-02-06}}</ref>

: <math>P_L = \frac{\sum (\frac{p_{c,t_n}}{p_{c,t_0}} \cdot E_{c,t_0})}{\sum E_{c,t_0}}</math>

This uses expenditure shares and price relatives, a practical approach when quantities are hard to measure, though it retains Laspeyres’ fixed-weight assumptions.<ref>Statistics New Zealand; ''Glossary of Common Terms'', [http://www2.stats.govt.nz/domino/external/omni/omni.nsf/0/7d11773239ed84eecc256f350016b841?OpenDocument "Laspeyres Index"]</ref>