Required section · Section 4 of 6

Guided calculation: one run, five results

Setting: a Level 2 sodium control is measured at 07:00, 07:08, 07:16, 07:24, and 07:32 on one generic indirect ion-selective electrode analyzer after stable startup. Same control lot, reagent lot, calibration status, operator, and run are stipulated, no specimen is involved, and the assigned target is 140.0 mmol/L. No laboratory acceptance limits or QC rule set are supplied for this teaching case, so no pass or fail judgment is possible from these numbers alone.

Mean: (139.8 + 140.1 + 140.0 + 139.9 + 140.2) / 5 = 140.0 mmol/L. Sample SD, using n minus 1: square each result's deviation from 140.0, sum the squares to 0.10, divide by 4, and take the square root, giving 0.16 mmol/L. CV: 100 times (0.16 / 140.0) = 0.11 percent. Bias against the 140.0 mmol/L target: 140.0 minus 140.0 = 0.0 mmol/L, or 0.0 percent.

The 07:32 result, 140.2 mmol/L, sits 1.26 sample SD above this five-result mean: (140.2 minus 140.0) divided by 0.16. That number describes distance in SD units for this specific run; it is not a rejection signal, because no acceptance limit or QC rule was declared for this teaching case, and a standardized distance never authorizes a release decision by itself.

These values describe this short, stipulated dataset only. Five results give an imprecise estimate of a true mean and SD, so this run cannot by itself establish long-term precision, confirm normality, or authorize releasing patient results tied to this control event, because the laboratory's own acceptance criteria are what would make that call, and none were supplied here.

Keep the unit attached to every number you report (mmol/L for mean, SD, and bias; percent for CV), and state which SD convention and which time window produced it.

A dot plot of five sodium control results from 07:00 to 07:32, ranging from 139.8 to 140.2 mmol/L, with a dashed teal mean line at 140.0 mmol/L, a light blue band spanning plus or minus one sample SD, and the final 07:32 result at 140.2 mmol/L marked in coral as the point 1.26 SD above the mean. — Figure 1Five results plotted in time order, with the mean and a plus-or-minus one sample SD band.

Level 2 sodium control, one run, 07:00-07:32
Time	Result (mmol/L)	Flag
07:00	139.8	none
07:08	140.1	none
07:16	140.0	none
07:24	139.9	none
07:32	140.2	none

Knowledge checks

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Knowledge check 1

Using the five 07:00-07:32 results (139.8, 140.1, 140.0, 139.9, 140.2 mmol/L), what is the mean, correctly rounded to match the reported precision?

Choose one option.

140.0 mmol/L

140.1 mmol/L

139.9 mmol/L

140.02 mmol/L, reported unrounded

Knowledge check 2

The 07:32 result sits 1.26 sample SD above the five-result mean. What does that number tell you by itself?

Choose one option.

How far that result sits from this short run's mean, in SD units; it does not by itself say whether the result is acceptable.

That the result exceeds the laboratory's rejection limit.

That the analyzer is out of calibration.

That this result should be excluded from later calculations.

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