### Glossary terms for 'A'

Accessible population | The group of people to whom the investigator has access and who could be selected for, or approached about participating in, the study. For example, the accessible population consisted of women with breast cancer who were treated within 6 weeks of their original diagnosis at Longview Hospital from January 1, 2013 through June 30, 2014. See also target population and intended sample. |

Accuracy | The degree to which a measurement corresponds to its true value. For example, self-reported bodyweight is a less accurate measurement of actual bodyweight than one made with a calibrated electronic scale. |

Adjustment | A general name for various statistical techniques used to account for the effects of one or more variables on an association between two other variables. For example, adjustment for income reduced the magnitude of the association between education and mortality. |

Alpha | When designing a study, the preset maximum probability of committing a type I error, that is, rejecting the null hypothesis when it is true. For example, by choosing an alpha of 0.05, the investigator set a maximum probability of 5% that her study would find a statistically significant association between non-white race and the risk of colon cancer by chance alone. Also called the level of statistical significance. |

Alternative hypothesis | The proposition, used in estimating sample size, that there is an association between the predictor and outcome variables in the population. For example, the study?s alternative hypothesis was that teenagers who smoke cigarettes have a different likelihood of dropping out of school than those who do not smoke. See also null hypothesis. |

Analytic study | A study that looks for associations between two or more variables. For example, the investigator did an analytic study of whether height was correlated with blood pressure in medical students. See also descriptive study. |

Association | A quantifiable relationship between two variables. For example, the study found an association between male sex and risk of cognitive impairment among 60 to 69-year-olds, with a risk ratio of 1.6.Before-after study. A study that compares the attributes of subjects before and then again after an intervention. For example, the study compared mean serum cholesterol levels before and after institution of a low-fat diet. |

Glossary material from Hulley SB et al. *Designing Clinical Research*, 4th ed. Philadelphia, Lippincott Williams & Wilkins, 2013.