alternative hypothesis

hypothesis /hy·poth·e·sis/ (hi-poth´ĕ-sis) a supposition that appears to explain a group of phenomena and is advanced as a basis for further investigation.

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alternative hypothesis  one that is compared with the null hypothesis in a statistical test.

biogenic amine hypothesis  the hypothesis that depression is associated with deficiency of biogenic amines, especially norepinephrine, at functionally important receptor sites in the brain and that elation is associated with excess of such amines.

jelly roll hypothesis  a theory explaining the formation of nerve myelin, which states that it consists of several layers of the plasma membrane of a Schwann cell wrapped spirally around the axon in a jelly roll fashion.

lattice hypothesis  a theory of the nature of the antigen-antibody reaction which postulates reaction between multivalent antigen and divalent antibody to give an antigen-antibody complex of a lattice-like structure.

Lyon hypothesis  the random and fixed inactivation (in the form of sex chromatin) of one X chromosome in mammalian cells at an early stage of embryogenesis, leading to mosaicism of paternal and maternal X chromosomes in the female.

null hypothesis  the particular one under investigation, which frequently asserts a lack of effect or of difference.

one gene–one polypeptide chain hypothesis  a gene is the DNA sequence that codes for the production of one polypeptide chain. Antibodies are an exception; separate genes for variable and constant regions are rearranged to code for a single polypeptide.

response-to-injury hypothesis  one explaining atherogenesis as initiating with some injury to the endothelial cells lining the artery walls, which causes endothelial dysfunction and leads to abnormal cellular interactions and initiation and progression of atherogenesis.

sliding filament hypothesis  the stretching of individual muscle fibers raises the number of tension-developing bridges between the sliding contractile protein elements (actin and myosin) and thus augments the force of the next muscle contraction.

Starling's hypothesis  the direction and rate of fluid transfer between blood plasma in the capillary and fluid in the tissue spaces depend on the hydrostatic pressure on each side of the capillary wall, on the osmotic pressure of protein in plasma and in tissue fluid, and on the properties of the capillary walls as a filtering membrane.

wobble hypothesis  one describing how a specific transfer RNA (tRNA) molecule can translate different codons in a messenger RNA (mRNA) template. It states that the third base of the tRNA anticodon does not have to pair with a complementary codon (as do the first two) but can form base pairs with any of several related codons.

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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 (H₀) that the effect under investigation does not exist and the alternative hypothesis (H₁) that some specified effect does exist, based on the observed value of a test statistic whose sampling distribution is completely determined by H₀. The decision is made to reject H₀ and by implication to accept H₁ 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 H₀ 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 H₁.

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alternative hypothesis

Epidemiology A hypothesis to be adopted if a null hypothesis proves implausible, where exposure is linked to disease. See Hypothesis testing. Cf Null hypothesis.