表中光照度数据为上午和下午两次观测值的平均值。

The data of solar radiation in the table were the average of forenoon and afternoon.

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对于随机的变化关键要记住的是，实际的观测资料都在平均数或者中间值周围。

The key thing to remember about random variables is that actual observations are centered around an average, or mean value.

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图1：正态概率分布函数，它表明实际的观测值可能会偏离期望值(或者平均值)。

Figure 1: Normal probability density function, which illustrates that actual observations may vary from the expected (or average).

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