# hypothesis

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## hypothesis

[hi-poth´ĕ-sis]a supposition that appears to explain a group of phenomena and is advanced as a bases for further investigation.

**alternative hypothesis**the hypothesis that is formulated as an opposite to the null hypothesis in a statistical test.

**complex hypothesis**a prediction of the relationship between two or more independent variables and two or more dependent variables.

**directional hypothesis**a statement of the specific nature (direction) of the relationship between two or more variables.

**Lyon hypothesis**a hypothesis about development of X chromosomes in the embryo; see lyon hypothesis.

**Monro-Kellie hypothesis**[mun-ro´ kel´e] an explanation of the maintenance of intracranial pressure: The skull is viewed as a closed container housing brain tissue, blood, and cerebrospinal fluid; a change in any of these three components will affect the other two. If the volume added to the cranial vault is equal to the volume displaced, the intracranial volume will not change.

**nondirectional hypothesis**a statement that a relationship exists between two variables, without predicting the exact nature (direction) of the relationship.

**null hypothesis**the hyothesis that the effect, relationship, or other manifestation of variables and data under investigation does not exist; an example would be the hypothesis that there is no difference between experimental and control groups in a clinical trial.

**hypothesis test**the abstract procedure that is the theoretical basis of most statistical tests. A hypothesis test decides between two hypotheses, the

*null hypothesis*(

*H*

_{0}) that the effect under investigation does not exist and the

*alternative hypothesis*(

*H*

_{1}) that some specified effect does exist, based on the observed value of a

*test statistic*whose sampling distribution is completely determined by

*H*

_{0}. The decision is made to reject

*H*

_{0}and by implication to accept

*H*

_{1}when the test statistic falls within a given set of values called the

*critical region.*This region is so determined that the probability of rejecting

*H*

_{0}when it is in fact true (a so-called

*Type I error,*the reporting as significant results that are only the result of random variation and not a real effect), is set at a specified level (symbol α). When this level is set before the data are collected, usually at 0.05 or 0.01, it is called the

*significance level*or

*α level*. It is now more common to report the smallest α at which the null hypothesis can be rejected; this is called the

*significance probability*or

*P value*. The ability of the test to accept a true alternative (and thus to detect a real effect when it exists) is termed the

*power*of the test. Note that no statistical test actually tests the

*H*

_{1}.

## hy·poth·e·sis

(hī-poth'ĕ-sis),A conjecture advanced for heuristic purposes, cast in a form that is amenable to confirmation or refutation by conducting of definable experiments and the critical assembly of empiric data; not to be confused with assumption, postulation, or unfocused speculation.

See also: postulate, theory.

See also: postulate, theory.

[G. foundation, assumption fr.

*hypotithenai,*to lay down]## hypothesis

(hī-pŏth′ĭ-sĭs)*n.*

*pl.*

**hypothe·ses**(-sēz′)

**1.**A tentative explanation for an observation, phenomenon, or scientific problem that can be tested by further investigation.

**2.**Something taken to be true for the purpose of argument or investigation; an assumption.

**3.**The antecedent of a conditional statement.

## hypothesis

Epidemiology A supposition, arrived at from observation or reflection, that leads to refutable predictions; a conjecture cast in a form that will allow it to be tested and refuted## hy·poth·e·sis

, pl.**hypotheses**(hī-poth'ĕ-sis, -sēz)

A conjecture cast in a form that is amenable to confirmation or refutation by experiment and the assembly of data; not to be confused with assumption, postulation, or unfocused speculation.

See also: postulate, theory

See also: postulate, theory

[G. foundation, assumption fr.

*hypotithenai,*to lay down]## hypothesis

A tentative proposition used as a basis for reasoning or experimental research, by means of which it may be rejected or incorporated into accepted knowledge. See NULL HYPOTHESIS.## hypothesis

a proposition assumed on the basis of observation which might account for or explain something which is not fully understood. see SCIENTIFIC METHOD.## significance

In statistics, an indication that the results of an investigation on a population (e.g. patients) differ from those of another population (e.g. general) by an amount that could not happen by chance alone. This is evaluated by establishing a

**significance level**, that is the probability, called*p***value**, which leads us to reject or accept the**null hypothesis**H_{o}(there is no significant difference between two populations and the difference is attributed to chance) and accept or reject the**alternative hypothesis**H_{1}that there is a statistically significant difference between two populations. A*p*value*p*< 0.05 is often considered significant, but the lower this figure, the stronger the evidence.*See*randomized controlled trial.## hy·poth·e·sis

, pl.**hypotheses**(hī-poth'ĕ-sis, -sēz)

A conjecture advanced for heuristic purposes, cast in a form that is amenable to confirmation or refutation by conducting of definable experiments and the critical assembly of empiric data.

See also: theory

See also: theory

[G. foundation, assumption fr.

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