A national health survey employs a complex design with 40 primary sampling units (PSUs), 2 replicate groups per PSU. Using Balanced Repeated Replication (BRR), the survey team constructs 40 replicate weights, each omitting one PSU pair and upweighting the rest.

For the estimate of mean systolic blood pressure $\hat{\mu} = 128.5$ mmHg:

• Replicate 1: 128.3
• Replicate 2: 128.7
• ...
• Replicate 40: 128.4
• Mean across replicates: $\bar{\hat{\mu}} = 128.5$

Sum of squared deviations: $\sum_{k=1}^{40}(\hat{\mu}_k - \bar{\hat{\mu}})^2 = 6.4$

The BRR variance estimate is: $V_{BRR} = \frac{1}{40}\sum_{k=1}^{40}(\hat{\mu}_k - \bar{\hat{\mu}})^2$

What is the approximate standard error of the blood pressure estimate?