A laboratory sample of gas is known to have a carbon monoxide concentration of 50 ppm. A spectrophotometer is used to take five independent measurements of this concentration. The five measurements, in ppm, are 51, 47, 53, 52, and 52. Suppose the spectrophotometer has been recalibrated, so we may assume that the bias is negligible. The spectrophotometer is now used to measure the CO concentration in another gas sample. The measurement is 56.9 ppm. How should this measurement be expressed? Round your final answers to one decimal place.
_____ (plus or minus)_____ ppm