[escepticos] el arte de torturar a los numeros hasta que dicen lo que tu quie

["carlitos" <180807@...>]

Hola, hola.

> [Francisco]
> da igual coger la porra con la izquierda o con la derecha....¡miento! un
> boxeador zurdo tienen inmensas ventajas cuando tiene delante a un
> boxeador diestro.

Por simetría, tendrá las misma ventajas que el diestro al enfrentarse
al zurdo. Eso en cuanto a geometría; otra cosa es que al estar
acostumbrado a pegarse con diestros un zurdo resulte más difícil.

> [Ernesto]
> > El único problema es que ser zurdo o diestro es uno de esos "escasos"
> > caracteres de los que se sabe que carecen de base genética.

¿En serio?  Una duda dudosa: en el caso de personas con los órganos
al reves (no por fuera, sino con el corazón a la derecha y tal), ¿hay
mayoría de zurdos?

Divagaciones aparte, el tema de los zurdos me ha recordado lo
siguiente:

Es "bien conocido" que los diestros viven mas que los zurdos.
Esto no es cierto (o la diferencia es minima). Si, cierto, las
estadisticas demuestran que la mayoria de los zurdos son jovenes.
Pero esto no quiere decir que mueran pronto por ser zurdos; lo que
ocurre es que hace an~os, en las escuelas se insistia en que los
zurdos se acostumbrase a usar su mano derecha.

Otro ejemplo de cómo malemplear la estadística.

Saludos, Carlos Ungil

PD: PARENTAL AGE GAP SKEWS CHILD SEX RATIO

   The proportion of male to female births increases during and
shorly after periods of war (1,2). We show that the age difference
between parents (age of husband-age of wife) predicts the sex of the
first child. We also find that in England and Wales, the mean spouse
age difference increased during and inmediately after the two World
Wars and was strongly correlated with the sex ratio during the period
1911-52.
   We obtained the age and sex of children from 301 families who
attended secondary schools that recruited from a wide range of
socioeconomic groups. The mean age difference Da (age of husband-age
of wife) was 2.48 years+-0.23 (s.e.m.) and there were 301 first-born
and 206 second-born children. Among first-borns there was an excess
i¡of daughters from couples with low Da and an excess of sons from
those with high Da
(Da = -9 to -1 years: 14 sons and 29 daughters;
Da = 0 to 5 years: 117 sons and 84 daughters;
Da= 5 to 15 years: 37 sons and 20 daughters;
x^2 = 11.86, P = 0.0027).
Among second-borns there was the opposite but non-significant
tendency
(Da = -9 to -1 years:22 sons and 11 daughters;
Da = 0 to 4 years: 93 sons and 89 daughters;
Da = 5 to 17 years: 20 sons and 25 daughters;
x^2 = 3.93, P = 0.14).
 The age of parents at the birth of the child has a weak effect on
the child´s sex (3). However, multiple regression analyses with sex
of child as the dependent variable and Da and age of mother as
independent variables showed that Da remained significantly
associated with sex of child
(Da / age of mother ---- Da: standardized partial regression
coefficient b1 = -0.14, t = 2.35, P = 0.02;
age of mother: b2 = 0.13, t = 0.22, P = 0.83;
Da / age of father ---- Da: b1 = 0.14, t = 2.34, P = 0.02;
age of father: b2 = 0.13, t = 0.21, P = 0.83).
 Local and national patterns of Da during this period 1911-52 (4) are
shown in Fig. 1a, c. If couples do not delay the birth of their first
child, Da and sex ratio should be correlated and changes in sex ratio
should be preceded by changes in Da. This is seen in 1914-18, but not
during the Second World War (Fig 1b, c). Registration of second and
subsequent births will weaken the relationship between Da and sex
ratio so that and exact correlation is unlikely. Nevertheless a
regression of sex ratio on Da shows that the latter explains 68% of
the variance of the former (Fig. 1d). Age of woman at marriage was
negatively related to the sex ratio (b = -0.003, r^2 = 0.23, F =
12.19, P = 0.001). However a multiple regression analysis with sex
ratio as the dependent variable and Da and bride's age as independent
variables left Da as the only significant correlate of sex ratio (Da:
b1 = 0.78, t = 8.26, P = 0.0001; age of bride: b2 = -0.14, t = 1.51,
P = 0.14).
 Rank in many animals is related to the sex of their offspring (5).
In humans, the elite often form partnerships with high Da (6) and
have more sons than daughters (7). It may be that during wartime
women prefer to marry older men with high resources and this leads to
an increase in Da. We do not know how the sex of first-borns is
adjusted in relation to Da. Women could influence the motiliy of
sperm bearing either X or Y chromosomes or they may invest
differentially in males and females in utero leading to higher
miscarriage rates of one or the other sex.

J.T. Manning, R.H. Anderton, M. Shutt
Population Biology Research Group, School of Biological Sciences,
University of Liverpool, Liverpool L69 3BX, UK
e-mail: jtmann en liv.ac.uk

(1) Martin, W.J.  Lancet 1, 807 (1943)
(2) MacMahon, B. & Pugh, T.F.  J. Hum. Genet. 6, 284-292 (1954)
(3) Bromwich, P.  Prog. Obstet. Gynaecol. 7, 217-231 (1989)
(4) The Registrar General's Statistical Review of England and Wales,
Part II Civil (HMSO, London, 1921-1952)
(5) Clutton-Brock, T.H. & Iason, G.R. Q.  Rev. Biol. 61, 339-374
(1986)
(6) Kenrick, D.T. & Keefe, R.C.  Behav. Brain. Sci. 15, 75-133 (1992)
(7) Mueller, U.  Nature 363, 490 (1994)

--- hasta aquí la transcripción (no voy a dibujar las gráficas) ---
ahora unos pocos comentarios:

¿Por qué para los primeros hijos se cuenta dos veces Da=5?
¿Cómo puede ser que una pareja con Da=17 tuviera un segundo hijo sin
tener previamente un primer hijo (el Da máximo es 15)?
¿Son fallos sin importancia?
¿Cómo de bien hecho puede estar un estudio en el que el numero de
primeros hijos es 168 y el de primeras hijas 133?  ¡Sex ratio = 1.26!
¿Cómo podría influir diferencia de edad para que las mujeres
favorezcan la fecundación o el desarrollo de uno de los sexos?
¿Por qué sólo sucede algo espectacular con el primer hijo?  ¿Vuelve a
invertirse la tendencia con el tercero?  ¿Es realmente extraño
encontrar una correlación después de probar tantas posibilidades?
¿Es realmente tan fácil publicar en Nature?  ¿Será cierto a pesar de
todo?