7个数据的A类不确定度计算方法及步骤

首先计算平均值：

$\bar{x} = \frac{1}{n}\sum_{i=1}^n x_i = \frac{189.8+189.4+189.6+189.9+189.7+190.0+189.5+189.7}{8} = 189.75$

然后计算每个数据与平均值的偏差：

$d_i = x_i - \bar{x}$

$d_1 = 189.8 - 189.75 = 0.05$

$d_2 = 189.4 - 189.75 = -0.35$

$d_3 = 189.6 - 189.75 = -0.15$

$d_4 = 189.9 - 189.75 = 0.15$

$d_5 = 189.7 - 189.75 = -0.05$

$d_6 = 190.0 - 189.75 = 0.25$

$d_7 = 189.5 - 189.75 = -0.25$

然后计算偏差的平均值：

$\bar{d} = \frac{1}{n}\sum_{i=1}^n d_i = \frac{0.05-0.35-0.15+0.15-0.05+0.25-0.25}{8} = 0$

因为平均偏差为0，所以a类不确定度为：

$u_a = \sqrt{\frac{1}{n-1}\sum_{i=1}^n (d_i-\bar{d})^2} = \sqrt{\frac{0.05^2+(-0.35)^2+(-0.15)^2+0.15^2+(-0.05)^2+0.25^2+(-0.25)^2}{7}} \approx 0.137$

因此，这组数据的a类不确定度为0.137。