Summary of data sources and issues
Assessor’s data
All years
The data contains duplicate entries for parcels containing multiple buildings. Each entry holds the full assessed value for the parcel. These were filtered out by removing lines with the identical Account Number and Total Assessed Value.
Column names vary from year to year; they were adjusted to match the 2016-2018 names.
2019
The year 2019 uses assessor’s data from the Cambridge Property Database FY16-FY19 dataset on the Cambridge Open Data Portal.
2016 - 2018
The years 2016-2018 use assessor’s data from the Cambridge Property Database FY16-FY18 dataset formerly on the Cambridge Open Data Portal. This dataset is no longer available on the portal.
Many of the 2016 and 2017 entries in this data set contain values in the wrong columns. The bad records are all in properties where the PropertyClass contains a comma, for example “Private College, University” or “Rectory, Parsonage”. I was able to correct the errors well enough to extract the data needed.
Columns used from this data: AccountNumber, GISID, AssessedValue, StateClassCode, PropertyClass, TaxDistrict, Address, ResidentialExemption.
2015
The year 2015 uses assessor’s data from the Assessing Building Information FY2015 dataset.
Some entries in this data set have GIS ID of 0. For these entries, GIS ID was computed as the first two portions of the Account Number.
59 properties of interest were not included in the 2015 assessor’s data. Assessed value for these properties was obtained from the 2015 tax bills available from the Pay or View Bills Online service of the City Finance Department.
Residential Exemption data was not available for 2015. For residential parcels, the number of 2016 exemptions was used to adjust the estimated tax.
Columns used from this data: Account Number, GIS ID, AssessedValue, State Use, Building Type, Neighborhood/ Market Area, Address.
2014
The 2014 assessor’s data available on the Open Data Portal was missing many properties of interest. 2014 tax bills are not available online so another source was used. An archived copy of the 2014 assessor’s data received from City staff contained the properties of interest. A list of properties obtaining the residential exemption was received separately from staff.
The assessor’s data for 2014 contained GIS ID but not Account Number so the residential exemption was subtracted after aggregating by parcel.
Columns used from this data: GIS ID, Total Appraised Parcel Value, State Use, Building Type, Location.
2013
The 2013 assessor’s data is from an archived copy of the 2013 assessor’s data received from City staff. Residential Exemption data was not available for 2013. For residential parcels, the number of 2014 exemptions was used to adjust the estimated tax.
Parcel data
Parcel data for all years is from the FY Parcels shapefiles available from the Cambridge GIS Department Assessing data page.
Tax rates and computation
Historical residential and commercial tax rates were received from the City Treasury Department. Properties were classified as residential or commercial based on the State Class Code, with codes starting with “01” or “1” classified as residential, properties with codes starting with “9” classified as tax exempt and not included, and other properties classified as commercial.
Tax was computed as Assessed Value × Tax Rate / 1000. For residential properties eligible for the residential exemption, the exemption amount for the year was subtracted from the computed tax.
Study area
The study area was derived from the FY2018 Parcel Map Index shapefile. Parcel maps were chosen to encompass the desired study area. For each year, parcels contained in the study area were included in the study. The map below shows the study area.
Copyright 2017 Kent S Johnson 
2018-11-03