The entire set of people, animals, events, or things that make up the group to which a researcher is interested in applying the results of her research

A population includes all members of a class or set from which a smaller sample may be drawn and about whom the researcher wants to draw conclusions.

Every member of the population being studied has an equal chance of being picked for inclusion in the study. 

Researchers are better able to generalize their findings to the population of interest if a random sample is used.

The research method that can identify cause-and-effect relationships between variables. 

It requires that the researcher systematically manipulate and control one or more variables and then observe how the research subjects or participants respond to this manipulation. The variable that is manipulated is called the independent variable.

Strengths: * Can make cause-and-effect relationships, * Researcher has control

Weaknesses: Sampling errors. Often hard to generalize to the real world.

Research conducted in order to describe relationships between two variables measured in a large sample of participants.

It involves measuring two (or more) variables in order to determine if they are related. If the value of one variable increases in value as the other also increases in value, this is known as a positive correlation. A negative correlation occurs when there is an inverse relationship between the variables measured; as the value of one increases, the value of the other decreases.

Strengths: * Can study real-world behavior, * Can determine relationships.

Weaknesses: Cannot determine cause-and-effect.

A method used in descriptive research studies that occurs in a natural setting that has not been manipulated by the researcher. The researcher systematically observes and records what occurs in an unobtrusive manner. This is done so that the behavior of the participants being tested is not altered. 

Strengths: Can gather information in its usual setting as it naturally occurs.

Weaknesses: Cannot determine cause-and-effect. Observer bias possible.

A descriptive method that is an in-depth study of a single subject. It can include interviews, observations, and test results.

Strengths: Intensive information can be gathered about individuals.

Weaknesses: Cannot determine cause-and-effect. Expensive and time consuming. May not be able to generalize information gathered to others. Biased sample possible.

A descriptive research method that requires the researcher to ask a group of people about behaviors, thoughts, or opinions. Data is collected through questionnaires or interviews.

Strengths: Large amounts of information can be gathered from many people in a relatively short period of time.

Weaknesses: Cannot determine cause-and-effect. Biased sample possible. Response bias possible. Survey questions might not be reliable or valid.

It is used to examine differences between different-aged subjects at one point in time. The researcher recruits two or more samples of participants of differing ages and measures them on the variable of interest. For example, a researcher interested in age differences in short-term memory capacity could administer the same memory test to thirty 10-year-olds and thirty 50-year-olds. One weakness of this design is that if the younger group performs better than the older group on the memory test, the group difference could be due to either differences in age or because they are from two different birth cohorts (generations). The researcher cannot conclude that memory capacity declines with age. However, the strength of this approach is that it is a quick and easy way to collect information on how different age groups differ at one point in time.

Strengths: Data show differences between different-aged people at one point in time. Relatively quick and inexpensive.

Weaknesses: Cannot show changes that occur with age. Findings may be subject to birth cohort effects.

Measures changes on a variable of interest in the same group of participants at several points in their lives. 

For example, a developmental psychologist interested in the development of memory capacity could recruit 30 participants who are 10 years old and measure them at Time 1 and again at Time 2 when they are 20 years old, and again at Time 3 when they are 30 years old. A strength of this approach is differences in performance can be interpreted as indicating changes that occur with age. One weakness is that it is time consuming and expensive to carry out. In addition, subject mortality (subject drop out) is a problem with this design. Findings may be biased if subject drop out has occurred. The participants who complete the study may differ in significant ways from those particpants who have dropped out. The participants who complete the memory study described above may be healthier or have better memory ability compared to those who dropped out.

Strengths: Data show how a sample of people change as they age.

Weaknesses: Time-consuming and expensive. May not be able to generalize the findings to other birth cohorts. May have the problem of subject drop-out.

A research method that combines the cross-sectional and longitudinal designs. This method can measure changes that occur with age, differences between people who are different ages at one point in time, and can also quantify birth cohort effects

It was created recently by Schaie to examine developmental changes in intelligence in adulthood. Schaie was concerned that there are sighificant differences in intelligence between birth cohorts or generations. He designed this methodology, which combined the cross-sectional and longitudinal designs, to examine cohort differences. 

Researchers have been able to quantify birth cohort effects using this design.

Strengths: Has the strengths of both the cross-sectional and longitudinal methods and can also measure birth cohort effects.

Weaknesses: Time-consuming and expensive. May have the problem of subject drop-out.

1. defining a research problem

2. proposing a hypothesis and making predictions

3. designing and conducting a research study

4. analyzing the data

5. communicating the results and building theories of behavior.