10% of teenagers have tried hard drugs by age 17

Almost a third of 17-year-olds have tried cannabis and one in 10 have tried harder drugs, such as cocaine, ecstasy and ketamine, with similar rates of experimentation regardless of parents' education level, finds a new study ...

Psychology & Psychiatry

Mean or nice? These traits could make or break a child's friendships

Not all friendships are created equal. Some friends get along; others struggle to avoid conflict. Conventional wisdom holds that the tenor of a friendship with someone who is nice differs from that with someone who is mean, ...

Psychology & Psychiatry

Frequent cannabis use by young people linked to decline in IQ

A study has found that adolescents who frequently use cannabis may experience a decline in Intelligence Quotient (IQ) over time. The findings of the research provide further insight into the harmful neurological and cognitive ...

Diseases, Conditions, Syndromes

Study examines mono, chronic fatigue syndrome in college students

Many college students fully recover from infectious mononucleosis (which is almost always caused by Epstein-Barr virus) within 1-6 weeks, but some go on to develop chronic fatigue syndrome, also called myalgic encephalomyelitis ...

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Longitudinal study

A longitudinal study is a correlational research study that involves repeated observations of the same items over long periods of time — often many decades. It is a type of observational study. Longitudinal studies are often used in psychology to study developmental trends across the life span, and in sociology to study life events throughout lifetimes or generations. The reason for this is that unlike cross-sectional studies, longitudinal studies track the same people, and therefore the differences observed in those people are less likely to be the result of cultural differences across generations. Because of this benefit, longitudinal studies make observing changes more accurate and they are applied in various other fields. In medicine, the design is used to uncover predictors of certain diseases. In advertising, the Communicus System, the design is used to identify the changes that advertising has produced in the attitudes and behaviors of those within the target audience who have seen the advertising campaign.

Because longitudinal studies are observational, in the sense that they observe the state of the world without manipulating it, it has been argued that they may have less power to detect causal relationships than do experiments. But because of the repeated observation at the individual level, they have more power than cross-sectional observational studies, by virtue of being able to exclude time-invariant unobserved individual differences, and by virtue of observing the temporal order of events.

Longitudinal studies allow social scientists to distinguish short from long-term phenomena, such as poverty. If the poverty rate is 10% at a point in time, this may mean that 10% of the population are always poor, or that the whole population experiences poverty for 10% of the time. It is not possible to conclude which of these possibilities is the case using one-off cross-sectional studies.

Types of longitudinal studies include cohort studies and panel studies. Cohort studies sample a cohort, defined as a group experiencing some event (typically birth) in a selected time period, and studying them at intervals through time. Panel studies sample a cross-section, and survey it at (usually regular) intervals.

A retrospective study is a longitudinal study that looks back in time. For instance a researcher may look up the medical records of previous years to look for a trend.

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