MCAT P/S

Experimental study

Researcher manipulates an independent variable and randomly assigns participants to conditions. Highest level of evidence for causation. Requires control group and random assignment

Quasi-experimental study

Resembles an experiment but lacks random assignment. Participants assigned to groups based on pre-existing characteristics or natural events. Lower internal validity than true experiment

Correlational study

Measures relationship between two variables without manipulation. Cannot establish causation. Produces a correlation coefficient

Observational study

Researcher observes subjects without intervening or manipulating variables. Includes naturalistic observation, case studies, surveys

Longitudinal study

Follows the same subjects over an extended period of time. Good for tracking developmental changes. Expensive and subject to attrition

Cross-sectional study

Compares different groups of people at a single point in time. Cheaper and faster than longitudinal but cannot track individual change. Cohort effects are a confound

Case-control study

Retrospective. Compares people who have a condition (cases) with those who do not (controls) and looks back at exposure history. Good for rare diseases. Susceptible to recall bias

Cohort study

Follows a group sharing a common characteristic (e.g. birth year/exposure) over time. Can be prospective or retrospective. Measures incidence and relative risk

Independent variable (IV)

The variable manipulated by the researcher. The presumed cause. e.g. drug dose

Dependent variable

The variable measured as the outcome. The presumed effect. e.g. symptom severity

Control variable

A variable held constant to prevent it from influencing the dependent variable

Confounder

A third variable that correlates with both the independent variable and dependent variable, creating a spurious association. Controlled by randomisation or statistical adjustment

Mediator

A variable that explains the mechanism by which the IV affects the DV. It lies on the causal pathway between IV and DV

Moderator

A variable that changes the strength or direction of the relationship between the independent variable and dependent variable. It interacts with the independent variable

Internal validity

The degree to which a study establishes a causal relationship between IV and DV. Threatened by confounds, selection bias, and attrition

External validity

The degree to which findings generalise to other populations, settings, and times. Threatened by non-representative samples and artificial lab conditions

Construct validity

The degree to which a test or measure actually measures the theoretical construct it claims to measure

Content validity

The degree to which a measure covers the full range of the construct being assessed

Criterion validity

The degree to which a measure predicts an outcome (concurrent or predictive validity)

Ecological validity

The degree to which findings from a study reflect real-world behaviour and settings

Test-retest reliability

Consistency of a measure across time. The same participants produce similar scores on repeated testing

Inter-rater reliability

Consistency of a measure across different observers or raters

Internal consistency

The degree to which items within a measure all assess the same construct. Measured by Cronbach's alpha

Random sampling

Every member of the population has an equal chance of being selected. Maximises representativeness and external validity

Stratified sampling

Population is divided into subgroups (strata) and random samples are taken from each. Ensures representation of key subgroups

Convenience sampling

Participants selected based on easy availability. Quick and cheap but highly susceptible to selection bias

Selection bias

Systematic difference between those who are selected for a study and those who are not. Threatens internal and external validity

Response bias

Systematic tendency of participants to answer questions inaccurately or dishonestly

Social desirability bias

Participants answer in ways they believe are socially acceptable rather than truthfully. Leads to over-reporting of positive behaviours and under-reporting of negative ones

Observer bias

Researcher's expectations or beliefs influence how they record or interpret data. Controlled by blinding

Hawthorne effect

Participants change their behaviour simply because they know they are being observed. A form of reactivity bias

Placebo effect

Improvement in outcomes due to participants' belief that they have received an active treatment rather than due to the treatment itself

Random assignment

Participants are randomly allocated to experimental or control conditions. Equates groups on known and unknown confounds. The key feature distinguishing true experiments from quasi-experiments

Control group

A group that does not receive the experimental treatment. Provides a baseline against which to compare the treatment group

Blinding

Participants are unaware of which condition they have been assigned to. Reduces response bias and placebo effects

Double-blinding

Both participants and researchers/assessors are unaware of condition assignment. Controls for both placebo effects and observer bias

Placebo

An inert substance or sham treatment given to the control group. Used to separate the psychological effects of treatment expectation from the active treatment effect

Mean

The arithmetic average of a data set. Sensitive to outliers

Median

The middle value when data are ordered. More robust than mean when outliers are present

Mode

The most frequently occurring value in a data set. Can be used with any scale of measurement

Standard deviation (SD)

The average amount by which individual scores deviate from the mean. Measures spread of data. Large SD = high variability

Variance

The square of the standard deviation. Measures the average squared deviation from the mean

Normal distribution

A symmetrical bell-shaped distribution where mean = median = mode. ~68% of scores within 1 SD; ~95% within 2 SD; ~99.7% within 3 SD

Positive skew

Distribution tail extends to the right. Mean > median > mode. Often caused by a small number of very high outliers

Negative skew

Distribution tail extends to the left. Mean < median < mode. Often caused by a small number of very low outliers

p-value

The probability of obtaining results at least as extreme as observed, assuming the null hypothesis is true. Not the probability that the null hypothesis is true

Statistical significance

Conventionally p < 0.05. The result is unlikely to have occurred by chance alone. Does not imply practical importance or large effect size

Confidence interval (CI)

A range of values within which the true population parameter is estimated to lie with a specified probability (usually 95%). A 95% CI means: if the study were repeated 100 times, ~95 of the intervals would contain the true value

Type I error (false positive)

Rejecting a true null hypothesis. Concluding there is an effect when there is not. Probability = alpha (typically 0.05)

Type II error (false negative)

Failing to reject a false null hypothesis. Concluding there is no effect when there is one. Probability = beta

Statistical power

The probability of correctly rejecting a false null hypothesis (detecting a real effect).

Power = 1 − beta.

Increased by larger sample size, larger effect size, and higher alpha

Correlation coefficient (r)

A measure of the strength and direction of the linear relationship between two variables. Ranges from −1 to +1. r = 0: no linear relationship. r = +1: perfect positive. r = −1: perfect negative

Positive correlation

As one variable increases the other increases. r > 0

Negative correlation

As one variable increases the other decreases. r < 0

Correlation does not equal causation

A correlation between two variables does not mean one causes the other. Third variable (confound) problem and directionality problem both apply

Operational definition

A precise, measurable specification of how a variable is defined and measured in a study. e.g. "stress" operationalised as cortisol level

Nominal scale

Categories with no inherent order. e.g. blood type/gender/ethnicity. Only mode applies

Ordinal scale

Ranked categories with no equal intervals between ranks. e.g. Likert scale/socioeconomic class. Mode and median apply

Interval scale

Equal intervals between values but no true zero. e.g. temperature in Celsius/IQ score. Mean/median/mode all apply

Ratio scale

Equal intervals and a true zero point. e.g. height/weight/reaction time/income. All statistical operations apply

Monocular depth cues

Depth cues requiring only one eye: linear perspective (parallel lines converge with distance) + relative size (farther objects appear smaller) + interposition (overlapping objects appear closer) + texture gradient (texture becomes finer with distance) + aerial perspective + motion parallax

Binocular depth cues

Depth cues requiring both eyes: retinal disparity (slightly different images on each retina; brain calculates depth from difference) + convergence (eyes turn inward for near objects; muscle feedback signals distance)

Place theory of hearing

Different frequencies activate different locations along the basilar membrane. High-frequency sounds activate the base; low-frequency sounds activate the apex. Explains perception of high frequencies

Frequency theory of hearing

The basilar membrane vibrates at the same frequency as the sound wave, and neurons fire at that rate. Explains perception of low frequencies (<1000 Hz). Combined with volley principle for mid-range frequencies

Gate-control theory of pain

Non-painful sensory input (e.g. touch/vibration) can close a neural "gate" in the spinal cord, reducing pain signal transmission to the brain. Explains why rubbing an injury helps. Large-diameter fibres close the gate; small-diameter fibres open it

Trichromatic theory (Young-Helmholtz)

Colour vision depends on three types of cones sensitive to short (blue), medium (green), and long (red) wavelengths. Mixing their activation produces all perceived colours. Explains colour blindness from cone deficiency

Opponent-process theory

Colour vision is processed by opposing pairs: red-green/blue-yellow/black-white. Explains afterimages (staring at red → see green afterimage) and why you cannot perceive reddish-green. Occurs at the ganglion cell level after trichromatic processing

Auditory localisation

Localising sound in space using interaural time difference (ITD): sound reaches the nearer ear first; used for low frequencies. Interaural level difference (ILD): sound is louder at the nearer ear; used for high frequencies

Specific phobia

Marked fear of a specific object or situation (e.g. spiders/heights/blood). Fear is disproportionate and triggers avoidance. Diagnosed when it causes significant distress or functional impairment

Social anxiety disorder

Intense fear of social situations involving scrutiny or potential embarrassment. Fear of negative evaluation. Avoidance of social performance situations

Panic disorder

Recurrent unexpected panic attacks (sudden surge of intense fear with somatic symptoms: palpitations/shortness of breath/dizziness/chest pain) plus persistent worry about future attacks or maladaptive behaviour change

Generalised anxiety disorder (GAD)

Excessive difficult-to-control worry about multiple domains (work/health/family) for ≥6 months. Associated with restlessness/fatigue/concentration difficulty/irritability/muscle tension/sleep disturbance

OCD

Obsessions: recurrent intrusive unwanted thoughts/urges/images causing distress. Compulsions: repetitive behaviours or mental acts performed to neutralise obsession-related anxiety. Compulsions provide temporary relief but reinforce the cycle

PTSD

Develops after exposure to actual or threatened death/serious injury/sexual violence.

Symptoms: intrusion (flashbacks/nightmares) + avoidance (of reminders) + negative cognitions/mood + hyperarousal (hypervigilance/exaggerated startle/sleep disturbance).

Duration >1 month

Major depressive disorder (MDD)

≥2 weeks of depressed mood or anhedonia plus ≥5 total symptoms (weight change/sleep disturbance/psychomotor changes/fatigue/worthlessness/concentration difficulty/suicidal ideation). Monoamine hypothesis: ↓serotonin/norepinephrine/dopamine

Bipolar I disorder

At least one manic episode (≥1 week of elevated/expansive/irritable mood + ↑energy + ≥3 symptoms: grandiosity/↓sleep need/pressured speech/flight of ideas/distractibility/↑goal-directed activity/risky behaviour). Depressive episodes common but not required for diagnosis

Bipolar II disorder

At least one hypomanic episode (like mania but less severe/shorter/does not cause marked impairment) and at least one major depressive episode. No full manic episodes

Schizophrenia

Positive symptoms: hallucinations (most commonly auditory) + delusions + disorganised speech/behaviour.

Negative symptoms: flat affect/alogia (poverty of speech)/avolition/anhedonia/asociality.

Cognitive symptoms: impaired working memory/attention/executive function.

Dopamine hypothesis: ↑mesolimbic dopamine

Cluster A personality disorders

Odd/eccentric. Paranoid PD (pervasive distrust) + Schizoid PD (detachment/restricted affect) + Schizotypal PD (magical thinking/odd perceptions/social discomfort)

Cluster B personality disorders

Dramatic/emotional/erratic. Antisocial PD (disregard for others' rights/lack of remorse) + Borderline PD (unstable relationships/identity/affect/impulsivity) + Histrionic PD (excessive emotionality/attention-seeking) + Narcissistic PD (grandiosity/lack of empathy)

Cluster C personality disorders

Anxious/fearful. Avoidant PD (social inhibition/feelings of inadequacy/hypersensitivity to rejection) + Dependent PD (excessive need to be cared for) + Obsessive-compulsive PD (preoccupation with orderliness/perfectionism/control)

Somatic symptom disorder

One or more distressing somatic symptoms plus excessive thoughts/feelings/behaviours related to the symptoms. Symptoms may or may not have an identified medical cause. Distinct from feigning illness

Illness anxiety disorder

Preoccupation with having or acquiring a serious illness in the absence of significant somatic symptoms. High health anxiety. Formerly called hypochondriasis

Dissociative identity disorder (DID)

Presence of two or more distinct personality states or identities with recurrent amnesia between them. Associated with severe early trauma

Dissociative amnesia

Inability to recall important autobiographical information, usually of a traumatic nature. May include dissociative fugue (sudden unexpected travel with identity confusion)

Autism spectrum disorder (ASD)

Persistent deficits in social communication and interaction + restricted/repetitive patterns of behaviour/interests/activities. Present from early development. Spectrum varies widely in severity

ADHD

Inattention (difficulty sustaining attention/easily distracted/forgetful) and/or hyperactivity-impulsivity (fidgeting/leaving seat/talking excessively/interrupting). Symptoms present before age 12 in ≥2 settings

Tolerance

Reduced response to a drug after repeated use; higher doses required to achieve the same effect

Withdrawal

Physiological and psychological symptoms that occur when a drug is discontinued after dependence has developed. Symptoms are often opposite to drug effects

Dependence

Physical dependence: body adapts to drug requiring it for normal function (tolerance + withdrawal). Psychological dependence: compulsive craving/use despite harm without necessarily physical dependence

Addiction

Chronic relapsing disorder characterised by compulsive drug seeking/use despite negative consequences. Hijacks the mesolimbic reward pathway

Identity achievement

Erikson/Marcia: the individual has explored options and made a firm commitment to an identity. The healthy resolution of the identity vs role confusion stage

Identity moratorium

The individual is actively exploring identity options but has not yet committed. A normative and healthy transitional state

Identity foreclosure

The individual has committed to an identity without exploration, typically adopting parents' or authority figures' values without question

Identity diffusion

The individual has neither explored nor committed to an identity. Associated with apathy or avoidance. Most concerning resolution of identity vs role confusion

Social identity theory (Tajfel and Turner)

People derive part of their self-concept from membership in social groups. Individuals are motivated to maintain positive social identity by favouring their in-group over out-groups. Basis for in-group bias and intergroup conflict

Self-categorisation theory

Extension of social identity theory: people categorise themselves and others into social groups (depersonalisation). The salience of a particular identity shifts depending on context

Personal vs social identity

Personal identity: unique individual characteristics (personality/personal history).

Social identity: self-concept derived from group memberships (nationality/gender/religion/race).

Both contribute to overall self-concept

Acculturation strategies (Berry)

How individuals relate to their heritage culture vs dominant culture.

Integration: maintain heritage + adopt dominant (best outcomes).

Assimilation: abandon heritage + adopt dominant.

Separation: maintain heritage + reject dominant.

Marginalisation: reject both. Leads to acculturative stress

Nuclear family

A household unit consisting of two parents and their dependent children. The dominant family form in Western industrialised societies