– Spatial analysis on consumer price index data in Turkey
Selin Tekin & Birkan Erguc, Turkish Statistical Institute
Consumer Price Index (CPI), widely known as “the inflation”, is one of the most popular statistics published by Turkish Statistical Institute. Surveyors conduct several visits in a month to predefined businesses throughout the country and collect prices of predefined goods and services to produce CPI. Tablet computers used for data collection which ensures that data is stored with relevant GPS coordinate and timestamp. Even though reliability of the CPI directly related to spatial and temporal accuracy of the source data, no prior analysis conducted for the former aspect. In this study, approximately 30.000 surveyor visit data that collected in the first half of 2020 were analyzed and their spatial accuracies were calculated. First, geocoding of the businesses done as the addresses of businesses were kept as plain text. Second, point features for both GPS coordinates and geocoded coordinates were created and erroneous GPS data removed. Third, a spatial analysis conducted to measure distance between (i) data collection point and its respective business, (ii) data collection points for the same business at different times. An acceptable limit defined as 1km and visits above this limit reported. The results show that the approx. 90% of data points were in acceptable limits. Further inspection of remaining data points revealed that geocoding errors, based on plain text addresses, led false positives. Oracle, PostgreSql/PostGIS, QGIS and ArcGIS software were used for the study.