Gemini Linkage Analysis

Linkage Analysis withLinkage Analysis with

Environmental Criminology Research Inc.

Vancouver, B.C., Canada

GeminiGemini

•Gemini is the linkageanalysis subset of ECRI’sRigel Workstation crimeanalysis system, offered asa separate product.

•Gemini also includes thecrime database, crimemapping, and temporalanalysis components ofRigel.

What is linkage analysisin Gemini?What is linkage analysisin Gemini?

•Establishing that a series ofcrimes have been committedby the same offender(s)

•Identifying all of the crimesthat are part of the series.

Why do linkageanalysis?Why do linkageanalysis?

•Identify and focus resources on the most prolific anddangerous serial offenders (the 20% of criminalsresponsible for 80% of the crimes).

•Improve the accuracy of analyses, leading to betterintelligence and quicker arrests.

•Clear more crimes by linking them to a serial offenderalready arrested.

Four Steps in Linkage AnalysisFour Steps in Linkage Analysis

1.Recognize a serial pattern: Human analyst

2.Find all possibly-related crimes: database search

3.Verify series: Matrix linkage

(count the factors in common)

4.Identify crimes most likely linked to series: Bayesianlinkage

GEMINIGEMINI

Matrix Linkage MethodMatrix Linkage Method

Crime / FactorCrime / Factor

F1F1

F2F2

F3F3

F4F4

F5F5

F6F6

F7F7

C1 (Smith)C1 (Smith)

C2 (Jones)C2 (Jones)

C3 (Baker)C3 (Baker)

C4 (Anderson)C4 (Anderson)

C5 (Williams)C5 (Williams)

Advantages:

•Simple to use

•Can show graphicalrepresentation

•Can detect multiple series

•Produces formaldocumented results

Linkage Factors:

F1: Window broken

F2: Jewelry stolen

F3: Weekday afternoon

F4: Gloves used

… etc

Matrix Linkage in GeminiMatrix Linkage in Gemini

•Choose or enter somelinkage factors.

•A Linkage Table iscreated automatically.

•Point and click in thetable to fill out anymissing answers(Yes/No/Unknown) foreach crime and eachlinkage factor.

To create a linkage matrix in Gemini:

Example linkage table from Gemini

Linkage Factors in GeminiLinkage Factors in Gemini

•In Gemini all linkage factors are stated inthe form of Yes/No questions, such as:

“Gun used?” or “Window pried?”

•The answer to each question may be Yes,No, or Unknown (not reported, impossibleto determine, etc.)

Where do Linkage Factors comefrom?Where do Linkage Factors comefrom?

•From you, and from other analysts like you.What factors caused you think somecrimes were linked in the first place? Turnthose into Yes/No questions.

•Gemini will remember linkage factors thatyou have used before for any given crimetype in its database. You can also sharelists of potential linkage factors betweenanalysts.

Where do linkage answers comefrom?Where do linkage answers comefrom?

•If the answer can be found in the crime datainput to Gemini, you can define a simplequery to determine the answer automaticallyfor all crimes, e.g.:

Location type = “Detached house” ?

•Answers not found in the crime databasemust be determined by reading the crimereports, or by conducting follow-up interviewsor investigation.

Matrix Linkage Analysis in GeminiMatrix Linkage Analysis in Gemini

•Once you have created a linkage table withanswers, just click the Matrix Linkage button toperform the linkage analysis and generate theresults.

•You can add or delete crimes, change thelinkage factors, or change the answers and re-run the analysis in a few seconds.

Some limitations of the MatrixLinkage MethodSome limitations of the MatrixLinkage Method

•No relative weighting of linkage factors -strong ones count the same as weak ones.

•“Unknown” treated the same as “notpresent”.

•Only gives a rough estimate of strength oflinkage.

Bayesian Crime LinkageBayesian Crime Linkage

•We apply Bayes Theorem from the field ofstatistics to answer the question:•We apply Bayes Theorem from the field ofstatistics to answer the question:

“What is the likelihood that any givencrime is linked to our series based onthe specific set of linkage factors weare using?”“What is the likelihood that any givencrime is linked to our series based onthe specific set of linkage factors weare using?”

The actual implementation is simple, andrequires only a little extra work from theanalyst.The actual implementation is simple, andrequires only a little extra work from theanalyst.

•When you initially identified a possible linkedcrime series, you established the pattern bychoosing the core set of crimes that youthink define the series.•When you initially identified a possible linkedcrime series, you established the pattern bychoosing the core set of crimes that youthink define the series.

•This core crime set allows you to identifyother potential linkage factors and estimatethe consistency of those factors in the series.•This core crime set allows you to identifyother potential linkage factors and estimatethe consistency of those factors in the series.

•You can refine your initial series and thelinkage factors as your analysis proceeds.•You can refine your initial series and thelinkage factors as your analysis proceeds.

The Starting Point –Your Initial Core SeriesThe Starting Point –Your Initial Core Series

•The percentage of crimes in the linked seriesfor which this factor is present we call theConsistency.•The percentage of crimes in the linked seriesfor which this factor is present we call theConsistency.

•The percentage of crimes of this general typefor which this factor would typically bepresent we call the Uniqueness.•The percentage of crimes of this general typefor which this factor would typically bepresent we call the Uniqueness.

Definition of Consistency andUniquenessDefinition of Consistency andUniqueness

We need two additional pieces of informationfor each linkage factor to do Bayesian linkage:We need two additional pieces of informationfor each linkage factor to do Bayesian linkage:

•One of the distinguishing characteristicsof a particular commercial burglary seriesis that this burglar very often gains entryby picking a door lock.•One of the distinguishing characteristicsof a particular commercial burglary seriesis that this burglar very often gains entryby picking a door lock.

•Entry via picked door lock is seen in 80%of the crimes initially identified as part ofthis series.•Entry via picked door lock is seen in 80%of the crimes initially identified as part ofthis series.

Consistency = 80%Consistency = 80%

•Lock picking skills are not commonamong commercial burglars in this city.This method of entry is seen only about5% of the time in general.•Lock picking skills are not commonamong commercial burglars in this city.This method of entry is seen only about5% of the time in general.

Uniqueness = 5%Uniqueness = 5%

Example of Consistency andUniquenessExample of Consistency andUniqueness

•You normally estimate Consistency from yourinitially identified core series (Gemini cancalculate it for you if you want).•You normally estimate Consistency from yourinitially identified core series (Gemini cancalculate it for you if you want).

•Uniqueness must be estimated based on yourown background knowledge or someresearch. But it need only be estimated oncefor your jurisdiction and for the type of crimeyou are analyzing – the estimate can besaved and re-used in the future for this factor.•Uniqueness must be estimated based on yourown background knowledge or someresearch. But it need only be estimated oncefor your jurisdiction and for the type of crimeyou are analyzing – the estimate can besaved and re-used in the future for this factor.

•These values are just best guesses. Theydon’t have to be very accurate to beuseful.•These values are just best guesses. Theydon’t have to be very accurate to beuseful.

Estimating Consistency andUniquenessEstimating Consistency andUniqueness

Likelihood RatiosLikelihood Ratios

•Once you have estimated Consistency andUniqueness, Gemini will instantly calculate alikelihood ratio weighting for each linkagefactor, and an overall linkage likelihood for eachcrime.•Once you have estimated Consistency andUniqueness, Gemini will instantly calculate alikelihood ratio weighting for each linkagefactor, and an overall linkage likelihood for eachcrime.

Bayesian Linkage ResultsBayesian Linkage Results

•Bayesian linkage produces a list of crimes indescending order of linkage likelihood. You cango down the list as far as a 1:1 likelihood(crime is as likely to belong to the series asnot). Below that the odds are against the crimebeing linked.•Bayesian linkage produces a list of crimes indescending order of linkage likelihood. You cango down the list as far as a 1:1 likelihood(crime is as likely to belong to the series asnot). Below that the odds are against the crimebeing linked.

•Once you have established a Linkage Pattern and alinked series, you will probably want to compare newcrimes to your series as they occur to see if they match.

•Gemini can import regular periodic

updates to the crime reports and

compare to existing linkage

patterns to see if any of the new

crimes are likely linked to any

ongoing series.

Updating the Linkage AnalysisUpdating the Linkage Analysis

Summary: Why GeminiLinkage Analysis?Summary: Why GeminiLinkage Analysis?

Produces better-quality series data for furtheranalysis, such as composite offenderdescription and geographic profiling.

Provides quantitative evidence for crimelinkage.

Allows new crimes to be quickly andautomatically evaluated for linkage to existingseries.

Helps to identify and confirm crime series.