2.35467

The selection of a confidence level for an interval determines the probability that the confidence interval produced will contain the true parameter value.

Common choices for the confidence level C are 0.90, 0.95, and 0.99. These levels correspond to percentages of the area of the normal density curve.

For example, a 95% confidence interval covers 95% of the normal curve -- the probability of observing a value outside of this area is less than 0.05.

I am not sure if this answers very specific to planning projects. From what i am aware it depends on the sample distribution.

Following Steps might help you:

• Calculate the point estimate of a sample to estimate a population proportion.

• Construct a confidence interval for a population proportion based on a sample population.

• Calculate the margin of error for proportions as a function of sample proportion and

size.

Above information was based on following links and please refer to the links for more information.

http://www.stat.yale.edu/Courses/1997-98/101/confint.htm

http://www.ck12.org/about/wp-content/uploads/2008/12/CK12_Prob_Stat_Advanced.pdf

Page 357

I agree with Pavan first you need to find your sample distribution, according to your data.

The confidence level is indeed an interesting question, with most probably no definitive good answer. Even if it is somewhat different to your question, you can have a look on the following paper, which contains some interesting thoughts...

Convergence of traffic assignments: how much is enough?

D Boyce, B Ralevic-Dekic, H Bar-Gera

Journal of Transportation Engineering 130 (1), 49-55

95%