The Ohio Journal of Science. Vol. XVI., No. 2 (December, 1915) — John Shaqi
The Ohio Journal of Science. Vol. XVI., No. 2 (December, 1915)Various
History
The Ohio Journal of Science. Vol. XVI., No. 2 (December, 1915)
Various
Natural history -- Periodicals; Science -- Periodicals
=The Origin.= The generating curve is the symmetrical normal
probability curve with origin at its center. Since x = 0 when t = 0,
the origin of the translated curve coincides with that of the base or
generating curve. The translated curve may not be symmetrical so that
the mean ordinate may not coincide with the modal ordinate. Because of
the relation between corresponding areas the ordinate at the origin
must continue to divide the area under the curve into equal parts, that
is, the origin and median always coincide.
=Determination of the Constants.= Since the exact position of
the median can not ordinarily be determined by inspection or direct
computation there are in reality four constants to be determined: the
distance between the median and the mean, a, κ and λ.
In determining the constants it is usual to compute the value of the
first four moments. The third and fourth moments are extensions of
the idea of the well known formulas for the first and second moments.
Denoting the moments about the median by μ, we have
1 ⌠+∞
μ_{1}^´ = ——— │ xy dx
N ⌡-∞
1 ⌠+∞
μ_{2}^´ = ——— │ x^{2}y dx
N ⌡-∞
1 ⌠+∞
μ_{3}^´ = ——— │ x^{3}y dx
N ⌡-∞
1 ⌠+∞
μ_{4}^´ = ——— │ x^{4}y dx
N ⌡-∞
where N is the total area under the curve.
The values of the μ’s are computed from the data[8] and equated to the
corresponding integrals which of course involve the four constants.
In this way four equations are obtained from which the values of the
constants may be determined. Since it is our present object to discuss
the solution only of these equations, merely the principal results will
be given.
The general form for the moments about the median of the
area under the translated curve is
1 ⌠+∞
μ_{n}^´ = ——— │ x^{n}y dx
N ⌡-∞
1 ⌠+∞ a^n(t + κt^2 + λt^3)^n
= —————— │ ———————————————————————— e^{-t^2/2} a(1 + 2κt + 3λt^2) dt
√(2π)N ⌡-∞ a(1 + 2κt + 3λt^2)
1 ⌠+∞
= —————— │ a^n(t + κt^2 + λt^3)^ne^{-t^2/2} dt.
√(2π)N ⌡-∞
On applying the two well known formulas:
⌠+∞
│ x^{2n + 1}e^{-x^2} dx = 0
⌡-∞
⌠+∞ 2n + 1 ⌠+∞
│ x^{2n + 2}e^{-x^2} dx = —————— │ x^{2n}e^{-x^2} dx,
⌡-∞ 2 ⌡-∞
the determination of μ_1´, μ_2´, μ_3´ and μ_4´ is reduced to a matter
of algebraic detail. Then on transferring to the arithmetic mean as
origin the values of μ_2, μ_3, and μ_4 can be determined in terms
of a, κ and λ. It is most convenient however, to make use of the
quantities β_1 = μ_3^2/μ_2^3 and β_2 = μ_4/μ_2^2 or rather β = β_1/8
and ϵ = (β_2 - 3)/12 and express the constants in terms of these
quantities. It is to be noted that both ϵ and β are zero for a normal
distribution, that is, for λ = κ = 0.
Omitting the detailed reduction[9] which is straightforward and direct,
we have
(1) μ´ = aκ
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