There is a very interesting example to explain this. 3 blind men were invited in a circus to ‘experience’ an elephant. One blind touches the legs and explains the elephant " as a trunk of a tree ". Second blind touches the tail and explains elephant as " a rope ". Third blind touches the trunk and declared, " both of you are wrong, elephant is like a big snake ".
Even more interesting is the unfortunate fact that there is no research on nature and extent of crime in Pakistan, so here the elephant " is nothing and nowhere ".
In contemporary Criminology, the question about how crime is measured and what those measurements reveal about the nature and extent of crime are very important issues. To answer these questions, it is necessary to find that how data are collected and what they mean.

PART I: To test various theories about why people commit crime?
One might record the kinds of offences committed by people of different ages. Other might count the number of crimes committed at different times of the year.
After collecting the data, it should be ordered in a purposeful way and a theory or a systematic set of principles are needed to make predictions from the data collected. Criminologists analyse these data and use their findings to support or refute theories.

Several theories are examined that explain why people commit crime:
One theory is that high crime rates result from the wide disparity between people’s goals and the means available to them for reaching those goals. Those who lack legitimate opportunities to achieve their goals try to reach them through criminal means.

HYPOTHESIS: Lower class individuals engage in more serious crimes and do so more frequently than middle class individuals.
Next criminologists would collect facts, observations, and other pertinent information -- called data -- on the criminal behaviour of both lower class and middle class individuals. A finding that lower class persons commit more crimes support the theory that people commit crime because they do not have legitimate means to reach their goals.

PART II: Second objective of measurement is to enhance our knowledge of the characteristics of various types of offences.
- Why are some more likely to be committed than others?
- What situational factors, such as time of day or type of place influence the commission of crime?

PART III: Third objective of measuring crime is that criminal justice agencies depend on certain kinds of information to facilitate daily operations and to anticipate future needs.
e.g. How many people are leaving jails?
How many are receiving prison sentences?
Answers to these questions will not only regulate the day to day arrangements as number of beds needed or requirement of the staff in the jails but it also gives answers to the questions related with legislative and policy-making matters as what effect does a change in law have on the amount of crime committed?

Surveys: It is a systematic collection of respondents answers to questions asked in questionnaires or interviews. Generally surveys are used to gather information about the attitudes, characteristics or behaviour of a large group of persons, who are called the population of the survey. Surveys by criminologists measure the amount of crime, attitudes towards the police or towards sentencing of dangerous offenders, assessment of drug abuse and fear of crime.
Instead of interviewing the total population under study, most researchers interview a representative subset of that population ___ a sample. If a sample is carefully drawn researchers can generalise the results from the sample to the population.

Experiments: An investigator introduces a change into a process and makes the measurements or observations in order to evaluate the effects of change. Most experiments are done in laboratories, but its possible to do them in real world or field, hence the name field experiment.

Participant and Nonparticipant Observation: Observation is the most direct means of studying behaviour. Investigators may play a variety of roles in observing social situations . In non-participant they simply observe the activities in everyday settings and record what they see.

Case Studies: It is an analysis of all pertinent aspects of one unit of study such as an individual, an institution, a group or a community. The sources of information are documents like life histories, biographies, diaries, journals, letters and other records

Using Available data in research: Besides collecting their own data, researchers often depend on secondary data collected by private and public organisations.

In course of research, criminologists encounter many ethical issues as,
Confidentiality……… should the results of interviews be published??
Should criminologists be immune to prosecution for their failure to disclose the names of their subjects??
When criminologists encounter these problems, they can rely on standards for ethical human experimentation.

Figure: Process of bringing crime to the attention of Police

Source: R.F.Sparks, H.G.Genn, D.J.Dodd

In order for a criminal act to be " known to the police ", the act first must be perceived by an individual. It must then be defined or classified as something that places it within the jurisdiction of criminal justice system and it must be reported to the police. Then it is redefined before recording the act as a crime known to the police (as shown in figure ‘1’. If any of the step is broken, crime is never discovered to begin with.

Part I and Part II Offenses:

Part I: Collectively called Index crimes. They are serious and tend to be reported to the police and can be used in combination as an index of changes over time. They are,
            Crimes against the person. (Criminal homicide, rape, robbery and aggravated assault
            Crimes against property. (Burglary, motor vehicle theft)

Part II: All other crimes except traffic violations are included in this and include fraud, embezzlement, weapon offences and simple assaults.

Crime Rate:
It can be calculated by following formula:

Crime rate = number of reported crimes X 100,000
     total population

 If we say that homicide rate is 10.2, it means there are 10.2 homicides for every 100,000 persons in the population under consideration.

Victimization Surveys:
It measures the extent of crime by interviewing the individuals about their experiences as victims.