Class prep Go to S:\classes\UEP_ENV Copy whole folder “American Community Survey Error Exploration” to your Desktop Make writable: right-click on folder => properties => uncheck read-only Class prep 1 Using Windows Explorer, go to the following folder: American Community Survey Error Exploration \AFF_data_tables\ Median_HH_Income_tract 2 Open: a a ACS_10_SF4_B19013_metadata.csv – this is the metadata file for the ACS data ACS_10_SF4_B19013_Med_HH_Income.xlsx – this is the data table (median household income) Today Census mapping basics review and questions Understanding American Community Survey margin of errors Calculating a reliability index (coefficient of variation or CV) Visualizing the CV on a map Questions about joining tables to geography? Federal Information Processing Standards (FIPS) Codes Area Name FIPS State Massachusetts 25 County Suffolk 25025 Tract 000601 25025000601 We JOIN the data table to the geography table using the common ID column Mapping Numbers Graduated color Graduated Colors…number of renters Graduated Symbols…number of renters Normalization (“divide by”) Number of population in rental units normalized by total population in occupied housing units Fraction of renters living in each tract out of total population in occupied housing units Using “Normalization” Normalize by means “divide by” Percentage – e.g., number of renters over total population in occupied housing Result is a fraction, e.g., .45 Fractions are translated into percentages by multiplying by 100 .45 = 45% Density – population normalized by area (e.g., sq mi, acre) Classes and Classification Methods Classes and Classification Methods Classification methods Details from ArcGIS 10.1 Help – standard classification methods 1. Natural breaks – good for skewed data 2. Equal interval, defined interval, and standard deviation – good for evenly distributed data to show differences 3. Quantiles - good for evenly distributed data to show relative difference (e.g., top and bottom 20 percentile 4. Geometric interval – compromise that attempts to have similar number of features in each class with intervals being roughly the same Classification methods Details from ArcGIS 10.1 Help – standard classification methods Equal interval Defined interval Natural breaks Quantiles Standard deviation Geometric interval Try them out! Which classification methods is best? Formatting numeric labels But make it better! Clutter and data speak! Clearer and cleaner Review Categories versus numbers Proportional versus graduated symbols Understanding classification methods No “right” method – explore Different methods => very different results Number of classes – hard to distinguish over 6 Understanding normalization (“divide by”) Mapping a particular area – two selection options: Select the town first, then perform select by location to get all tracts that intersect that town (or have their centroid in that town) Zoom into an area slightly larger than the region you want to map, then interactively select all the tracts from in that area (e.g., use the select tool to make a box around them) Then Create Layer from Selected Features Copying and pasting the same layer in your table of contents If you want to map several variables that are within the same joined table(s), you can simply copy and paste the layer so that you have another copy Then create maps from a different variable in each layer Make sure to change title, legend American Community Survey What users need to know Test: why do we need to use ACS data in policy / environmental analysis? Because it has important information about our communities… Because it has important information about our communities… So we need to learn to use the information reliably… And especially to understand the margin of error for ACS estimates Review – What is the ACS? American Community Survey A continuous monthly survey of households Long set of questions covering many topics Data is released once a year 1 Year averages – areas with a population 65,000+ 3 Year averages – areas with a population 20,000+ 5 Year averages - all other areas (including census tracts and blockgroups) E.g., average number of people commuting by bicycle for 2007-2011 Use Census 2010 data where possible because it is 100% survey, thus has smaller sampling error Population Counts Age Race / Hispanic Ethnicity Housing Unit Counts and Tenure (rented, owner-occupied) Household and Family Relationships ACS: Use the highest aggregation you can in terms of tables (can be hard to find) ACS and Margin of Error Workers 16 and Over Means of transportation for commute – Tract Level - ACS 2005-2009 5 year estimates Universe is workers 16 and over Open the Excel files… a a ACS_10_SF4_B19013_Med_HH_Income.xlsx – this is the data table (median household income) ACS_10_SF4_B19013_metadata.csv – this is the metadata file for the ACS data Metadata file and data table… So let’s understand the margin of error… What is Sampling Error? Definition The uncertainty associated with an estimate that is based on data gathered from a sample of the population rather than the full population 39 Illustration of Sampling Error Estimate average number of children per household for a population with 3 households living in a block: Household A has 1 child Household B has Household C has 2 children 3 children The block average based on the full population is two children per household: (1+2+3)/3 40 Conceptualizing Sampling Error Three different samples of 2 households: 1. 2. 3. Households A and B (1 child, 2 children) Households B and C (2 children, 3 children) Households A and C (1 child, 3 children) Three different averages based on which sample is used: 1. 2. 3. (1 + 2) / 2 = 1.5 children (2 + 3) / 2 = 2.5 children (1 + 3) / 2 = 2 children 41 Sampling Error Census 2010 is a 100% survey so has smaller error ACS data is based on samples – error is larger The smaller the geography, the larger the error (because the sample is smaller) Especially true for variables that sample a small number of people, e.g., bike commuters ACS and Margin of Error Workers 16 and Over Means of transportation for commute – Tract Level - ACS 2005-2009 5 year estimates Universe is workers 16 and over American Community Survey and sampling error The margin of error is calculated and included with each estimate Calculated at 90% confidence level What does that mean? ACS and Margin of Error Workers 16 and Over Means of transportation for commute – Tract Level - ACS 2005-2009 5 year estimates Universe is workers 16 and over Confidence level of 90% We don’t know for sure how many people in Tract 3.02 take public transit to work Based on the ACS sample, our estimate over 5 years is that an average of 747 people take transit, +/- 226 at 90% confidence level If we did many, many samples of that same tract, 90% of the time the resulting range (521973 people) would contain the real number of commuters taking transit. 10% of the time it would not Confidence level of 90% The confidence level of a margin of error indicates the likelihood that the true population value (real number) falls within the margin of error We can be 90% confident that somewhere between 571 and 973 people take transit to work in tract 3.02 Also we know that Tract 3.02 has somewhere between 1958 and 2684 workers) So maybe half the workers take transit, or maybe just a fifth of them do. Ugh!!! If using ACS data, pay attention to margin of error! ACS table from American Factfinder…. Use metadata file plus AFF web site This table is showing Educational Attainment for universe of people 25 years and older Use AFF web site plus metadata file Bottom line for ACS More up to date information Continuous versus point in time measurement 5 year estimates are the most reliable because they have the largest samples But… Poorer precision at finer scales (e.g., census tract) or areas of low population (rural areas) Poorer precision for variables with low numbers (e.g., people who bike to work) Don’t go any lower than tracts for mapping ACS data Geographic Hierarchy Measures associated with sampling error 56 Look at Excel file for Med_HH_Income Measures Associated with Sampling Error Standard Error (SE) Margin of Error (MOE) Coefficient of Variation (CV) 58 Standard Error (SE) Definition A measure of the variability of an estimate due to sampling Depends on variability in the population and sample size Formula SE = MOE / 1.645 (for 90% confidence level) 59 Margin of Error (MOE) Definition A measure of the precision of an estimate at a given level of confidence (90%, 95%, 99%) MOEs at the 90% confidence level are published for all ACS estimates 60 Coefficient of Variation (CV) Definition The relative amount of sampling error associated with a sample estimate A measure of reliability Formula CV = Standard Error / Estimate * 100% 61 CV% is a measure of reliability. So what is a good CV %? No agreement Depends on purpose Census case studies: less than 15% may be reliable 15-30% - not reliable, be very careful Over 30% - not reliable, use with extreme caution To calculate CV, we first calculate the SE: SE = (MOE / 1.645) Then the CV% formula is: CV = (SE / estimate)*100 Two examples Median household income and biking to work Why do you think median household income generally show lower CVs (more reliable estimates)? Exploring Error and the American Community Survey Your turn! The American Community Survey Margin of Error Tutorial goes through all this, so do this on your own time for practice Census data table modifications Preparing data takes understanding and time Probably best to do it in Excel ahead of time Always remember to process the GeoID2 field to make it text To be compatible with shape file: Column names – 10 characters max, no spaces or symbols Close Excel tables before opening ArcMap From desktop, open the following mapfile American Community Survey Error Exploration \ Exploring Error in the American Community Survey.mxd Showing in ArcMap Join the fixed Household Median Income table to Census Tract shape file Create a map of Household Median income – 5 classes by quantiles Right-click and copy tract layer Right-click on Layers and choose Paste Layers Map CV – 3 classes, with breaks at 15, 30, and max value Symbolizing CV with hatch patterns Hands on exploration of commute data GIS Tools for Mapping ACS Estimates and Data Quality Information http://gesg.gmu.edu/ For your census mapping assignment You need to make 6 maps 6 different census variables (not necessarily from 6 different tables) At least two of the maps have to show ACS variables You don’t have to show CV on your maps but if you want to experiment, it’s good practice! For your census mapping assignment You can use census data you find from GIS clearinghouses – e.g., MassGIS Instructions for clipping coastal tracts on GIS Tips and Tutorials web site ACS and Error Always be aware of error Have a statement about error if you are making maps Might be good to visualize the CV as well, at least as an inset? In tables, include the margin of error It’s your reputation that’s at stake!