... eσμX2πσ1Y221⎟⎠⎞⎜⎝⎛−=− The quantity Y, which is the height of the curve at any point along the scale of X, is known as the probability density of that particular value of the variable quantity, X. Example 2.1: ... electricity lamps in the streets of the city. If these lamps have an average life of 1,000 burning hours, with a standard deviation of 200 hours, what number of the lamps might be expected to ... corresponding value of t = [(900 – 1000)/200] = -0.5. Entering the table with this value of t, we find for the probability of failure below 900 hours: P = 0.309. z The number of lamps, which will...