What Is the Body in a Unit Normal Table?
Unit normal tables split the curve at your z score into two regions with names: the body — the larger slice, on the mean's side of the cut — and the tail — the smaller slice beyond it. The table's classic columns report the area between the mean and z, and the area in the tail; "body" questions are asking you to navigate exactly those columns.
For z = 1.00: area between mean and z = 0.3413, area in the tail beyond z = 0.1587, and the body — everything on the mean's side, the area to the left here — totals 0.5 + 0.3413 = 0.8413. Three numbers, one cut, and every unit-normal question is an arrangement of them.
The columns, once and for all
The traditional unit normal table (the one in psychology and education statistics texts) carries, for each z: the area between the mean and z, and the area beyond z in the tail. For z = 1.96 those read 0.4750 and 0.0250. Notice they sum to 0.5 — they are two halves of one side of the curve. The cumulative probability our main z score table lists is then one addition away: for a positive z, Φ(z) = 0.5 + (mean-to-z area); for the same z, the tail column is already the right-tail probability. The dedicated unit normal table page lays all three columns side by side so the identities can be seen rather than memorised.
Where "body" gets slippery — negative z
Cut the curve at z = −1.00. The small slice beyond it on the left (0.1587) is the tail; the body is everything else — 0.8413 of the area, now lying to the right of the cut. Students hunting for "the body" in the mean-to-z column find 0.3413 and stop, but that column gives only the strip between the mean and z; the body for a negative z is 0.5 + 0.3413 measured from the other side. The rule that survives every case: the body is the larger portion, the tail the smaller, and the cut is at |z| — areas depend on the distance, the labels on the geometry. Examination questions phrase it exactly this way ("what proportion lies in the body?") precisely to test whether the picture, not the formula, is doing the work. Sketch the curve, mark the mean, cut at z, shade the larger piece: the number follows.
Related questions
Frequently asked questions
What is the tail in a unit normal table?
The smaller region beyond z, away from the mean: for z = 1.00, the tail is 0.1587 (15.87% of the distribution). Tail areas are the raw material of p-values and percentile extremes; body areas are the raw material of "typical range" statements.
Do body and tail always sum to 1?
Yes — they partition the whole curve at the cut, so body + tail = 1 for any z. The mean-to-z and tail columns sum to 0.5 for the same reason applied to one half. Either identity is a free arithmetic check on any unit-normal problem.
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