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SIO Corner: Hypotheses And Theories

This week the minefield of Hypotheses and Theories, the downfall of several famous investigations are the focus of our attention

In this series we will look at various aspects of life as an SIO. This includes the necessary skill sets for the successful SIO, the management of serious crime investigation and specific elements of investigative practice from initial response through crime scene examination and investigative strategies to dealing with suspects and the media. The articles are excerpts from the 2nd edition of Blackstone's, the 'Senior Investigating Officers' Handbook', written by two highly experienced SIOs (see 'About the Authors' at the end of the article).

|STARTHTML|Developing Hypotheses and Theories|ENDHTML|
Decisions may have to be based on or guided by an hypothesis. An hypothesis is defined as ‘a proposition made as a basis for reasoning without the assumption of its truth’; or ‘a supposition made as a starting point for further investigation from known facts’ (Oxford English Reference Dictionary, 2nd edn, OUP).
Developing and using hypotheses is a widely used technique that can be used to try to establish the most logical or likely explanation, theory, or inference for what may have occurred. Adhami and Browne, ‘Major Crime Enquiries: Improving Expert Support for Detectives’ Police Research Group Special Interest Series, Paper 9 (London: Home Office, 1996) referred to the inferential processes of detectives as a series of ‘if-then’ rules (involving a sentence that begins ‘If . . .’ closely followed by: ‘then . . . ’).

Ideally there should be sufficient reliable material available on which to base the hypotheses such as the sex and age of the victim, likely type of offender, modus operandi, location, type of attack, time of day, motive, crime scene interpretation etc. Knowledge and experience of previous cases will also assist in generating hypotheses. Generating and building hypotheses is an obvious and natural activity for an SIO, particularly during the initial stages of an investigation which is likely to commence once notified of the incident. SIOs need to begin to quickly assess information received about a crime in order to be able to generate possible theories and inferences about what they will be or are dealing with.
This is a process that forms the basis of early decision making and provides options that can be considered when applying the aforementioned problem-solving skills. As the enquiry progresses the technique can be tailored to approach a particular theme or part of an investigation where it is good practice to set out a declared objective—for instance, establishing how an offender may have come into contact with the victim.
The SIO can involve members of their enquiry team in the process to assist with this task and any other useful expert advisers such as scientists, pathologists, geographic profilers, behavioural investigative advisers, etc. This is also an important task for an analyst to assist with and can link into what they may be preparing on inference development and the identification of intelligence gaps that will help test the validity of the hypotheses and inferences (see Major Incident Analysis Manual (NCPE, 2005), 100–2 and Practice Advice on Analysis (ACPO NPIA, 2008), 79–83).

Clearly if there is sufficient information or evidence already available then there will be no need to use the hypothesis method. When limited information is available a problem-solving approach to decision making will be key, although the risks of making decisions in the absence of information should be recognized.

Initial information from the first response can help create tentative hypotheses and provide initial investigative direction when there is little information to go on. The term ‘keeping an open mind’ should be remembered together with the rule that it is better to gather as much information as possible before placing too much reliance on any one particular speculative theory. ‘It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts’ (A Conan Doyle, The Celebrated Cases of Sherlock Holmes (Octopus, 1981)).

The Practice Advice on Core Investigative Doctrine (NCPE, 2005), 73) sets out a checklist of considerations that have to be met when building hypotheses, which have been added to the list below:
|STARTHTML|Checklist—Hypotheses Building|ENDHTML|
|STARTHTML| • Beware of placing too much reliance on one or a limited number of hypotheses when there is insufficient information available.
• Remember the maxim of ‘keeping an open mind’.
• Ensure a thorough understanding of the relevance and reliability of any material relied upon.
• Ensure that hypotheses are kept under constant review and remain dynamic—remembering that any hypothesis is only provisional at best.
• Define a clear objective for the hypothesis.
• Only develop hypotheses that ‘best fit’ with the known information/material.
• Consult with colleagues and experts to discuss and formulate hypotheses.
• Ensure sufficient resources are available to develop or test the hypotheses.


|STARTHTML|The following hypotheses were initially considered:|ENDHTML|


The totally naked body of a female aged 17 was discovered in the rear alleyway of an area frequented by prostitutes and clients. Two local petty thieves found the body and reported it to the police. The victim was well known to the local police and often used drugs. There were no visible signs of injury other than what appeared to be needle marks on her arms and some small circular marks on her face. The body was lying on the ground in a supine position in some dirt and there were no traces of her clothing. A post-mortem examination revealed she had been strangled by unknown means and died of asphyxiation. There was no evidence of a sexual attack. It was unknown as to how the victim had ended up at the location or what exactly had happened, and unclear why she had no clothing on.

Local sex workers supplied information stating they never completely undressed for clients as it was too dangerous and limited means of quick escape. The pathologist was of the opinion that there were very few or no defensive marks on the body. The crime scene examiners and forensic scientists were initially unsure about where the murder took place.


  1. If their initial accounts prove to be inconsistent or unreliable then the two local thieves who found the body may have committed the murder.
  2. If the murder had taken place in nearby flats habitually used by drug abusers and prostitutes then the offender or someone else who found the body may have panicked and deposited it in the adjacent alleyway.
  3. The victim may have been murdered at the location where the body was found.
  4. If the murder took place elsewhere then this could have been a deposition site.
  5. If she had been murdered by a client who was having sex with her at the same location then he/she may have removed and took the clothing to destroy trace evidence, or retained it as a ‘trophy’.
  6. If a vehicle was involved and the victim was engaging in a sexual encounter as a prostitute with a client then the murder may have occurred after she had voluntarily or forcibly been made to remove her clothing and the murderer made off leaving her body at the side of the alleyway where it had been parked taking her clothes with him to dispose of incriminating evidence.
  7. The victim may have been murdered by a friend, boyfriend, or relative for reasons unknown.
  8. If the death was unintentional then it may have occurred during some sexual act such as autoeroticism and her partner panicked leaving her body in the alleyway
  9. If there was no vehicle involved then she had been in the alleyway voluntarily on foot with a client before she was killed.

Any one or a combination of these hypotheses was initially considered feasible by the SIO. Producing a list as comprehensive as this actually demonstrated the SIO was keeping an open mind and not concentrating too heavily on one or two possible theories. As the enquiry progressed and more material and information became available, a number of the hypotheses could be ruled out.

The outcome was that tyre impressions were later discovered at the scene and it transpired that a vehicle had been used by the offender. The victim had been strangled after sexual intercourse with a client in his car. The actual motive for the murder was never clear but the victim was known to be naive enough to be coerced into taking all her clothes off. The body had been dragged out of the car and left in the alleyway, and clothing was later deposited in a skip away from the scene, which was later found on CCTV footage. Through a process of interviewing previous clients and victim associates, a possible offender was eventually traced and arrested. The tyres of his vehicle matched those at the scene and some fibres from the victim’s clothing were found in the boot of his car. After a reconstruction the car was matched with the grainy image captured on CCTV, with a figure putting what looked like clothing into a waste disposal skip. After a full trial he was convicted and sentenced to life imprisonment.

|STARTHTML|Note: The circular marks on the victim’s face were heroin abuse scabs, and not rodent or insect bite marks or cigarette burns, as first suggested by some experts.|ENDHTML|

|STARTHTML|Adequacy and Assertion in Decision Making|ENDHTML|

There is an important difference between adequacy and assertion and this affects how they relate to decision making. This is because assertions do not carry the same weight as ‘adequate arguments’. Assertions are statements or declarations that do not necessarily have to contain essential supporting evidence, explanation, or reasoning. SIOs should always try to construct and rely upon adequate arguments rather than merely assertions, although the latter may be useful sometimes provided they are treated for what they are.This is important when understanding the construction and recording of decisions made. Adequate decision making should ideally be based upon sound reasoning with supportive argument based upon the information gathered. The SIO must not only identify but also support and evaluate different arguments and theories. The golden rule is that good arguments are backed by evidence, supporting material, and logical reasoning. It is also worth remembering the legal evidential test which in all criminal cases is ‘beyond reasonable doubt’. The SIO should pose the question: ‘Has the theory or hypothesis been adequately established and logically reasoned, and can it be maintained with confidence?’In other words, it is necessary to test the adequacy of the material and reasoning that supports any theory or decision that is chosen and to make an appropriate record of the process.

|STARTHTML|About the Authors: Detective Superintendent Tony Cook was a CID officer with Greater Manchester Police for over 31 years until his retirement in 2009. During his time as an SIO, he led a number of high profile investigations including operations into gangland violence at Moss Side, the Bolton murder of a teenage girl in 2002, and the Denton strangling case. He was a trained assessor for promotion and a qualified Authorising Officer under RIPA. Tony received 14 commendations and a first-class BSc Honours degree in social sciences and a Diploma in Social Policy & Criminology from the Open University. Andy Tattersall, formerly Detective Superintendent in Greater Manchester Police on the Force Major Incident Team, retired in 2007 after 33 years service and became the first ever Support Staff SIO in charge of a new Homicide Support Unit. With over 29 years in CID at all ranks Andy received the Homicide Working Group National Award for his Outstanding Achievement in the Field of Homicide in November 2006.|ENDHTML|

To see more details about the Senior Investigating Officers' Handbook, or to purchase a copy, click here

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