We noted that the normal curve is symmetrical. Since it is a symmetrical distribution the same percentage of cases falls within a certain range above the mean as falls within the same range below the mean.
In other words, if 68 percent Of all cases fall within 1 standard deviation unit either side of the mean, half of this (34.13%) will fall above the mean, and the other half (34.13%) below the mean. For group 3 in Table 6.1, this will imply that 34 people will be between 178 cm and 180 cm tall, and another 34 will be between 180 cm and 182 cm tall.