和訳をお願いします
Confounding is a central issue for epidemiologic study design. A simple definition of confounding would be the confusion, or, mixing, of effects: this definition implies that the effect of the exposure is mixed together 「ア」the effect of another variable, leading to a bias. Let us illustrate confounding with a classic example: the relation between birth order and the occurrence of Down syndrome. Figure 1 shows data on birth order and Down syndrome from the work of and Mantel.
These data show a striking trend in prevalence of Down syndrome with increasing birth order, which we might describe as the effect of birth order on the occurrence of Down syndrome.
(1)(The effect of birth order, however, is a blend of whatever effect birth order has by itself and the effect of another variable that is closely correlated with birth order.) The other variable is the age of the mother. Figure 2 gives the relation between mother’s age and the occurrence of Down syndrome from the same data. It indicates a much stronger relation between mother’s age and Down syndrome.
Because birth order and the age of the mother are highly correlated, we can expect that mothers
giving birth to their fifth baby are , as a group, considerably {イ} than mothers giving birth to their first baby. Therefore, the comparison of high-birth-order babies with lower-birth-order babies is to some extent a comparison of babies born to older mothers with babies born to younger mothers. Thus, the birth-order comparison in Figure 1 mixes the effect of mother’s age with the effect of birth order. The extent of the mixing depends on the extent to which mother’s age is related to birth order. (2) ( A good way to resolve the extent to which one variable’s effect explains the apparent effect of the other is to examine both effects simultaneously.) Figure 3 presents the prevalence of Down syndrome at birth by both birth order and mother’s age simultaneously.
(3) (Figure 3 shows clearly that within each category of birth order, looking from the front to the lack, there is the same striking trend in prevalence of Down syndrome with increasing {ウ}. In contrast, within each category of mother’s age, looking from left to right, there is {エ} discernible trend with {オ}.)
Confounding: 交絡（疫学用語） epidemiologic study:疫学研究 exposure:曝されること、曝露 bias:偏り variable:変数 Down syndrome:ダウン症候群 order:順位、順序 discernible:認められる、識別できる prevalence: 有病率
（図1,2,3はアップ出来ませんでした。申し訳ありません。）
(2)のかっこでくくってある英文の和訳がよく分かりません。僕の和訳は直訳通り、「ある一つの変数の影響が別の変数による明らかな影響をはっきりさせる程度を解決するよい方法は、両方の影響を同時に調べることだ。」と考えましたが、意味不明です。
長い文ではありますが、よろしくお願いいたします。
