Ok so my question is how do I determine which confidence level (%) to use? I und

Question

Ok so my question is how do I determine which confidence level (%) to use? I understand how to find the degrees of freedom no problem but I dont understand how my instructor sometimes uses a 90% confidence level value and other times 95% confidence level value. How do you know which one to use after finding degrees of freedom?

Table 4-2 Values of Student's t Confidence level (%) Degrees of freedom 50 90 95 98 99.5 99.9 1.000 6.314 12.706 31.821 63.656 127.321 636.578 0.816 2.920 4.303 6.965 9.925 14.089 31.598 0.765 2.353 3.182 4.541 5.841 7.4 0.741 2.132 2.776 3.747 4.604 5.598 8.610 0.727 2.015 2.571 3.365 4.032 4.773 6.869 0.718 .943 2.447 3.143 3.707 4.317 5.959 0.711 1.895 2.365 2.9983.500 4.029 5.408 0.706 1.860 2.306 2.896 3.355 3.832 5.041 0.703 1.833 2.262 2.821 3.250 3.690 4.781 0.700 1.812 2.228 2.764 3.169 0.691 1.753 2.131 2.602 2.947 0.687 1.725 2.086 2.528 2.845 3.153 3.850 0.684 1.708 2.060 2.485 2.787 3.078 3.725 0.683 .697 2.042 2.457 2.750 3.0303.646 0.681 .684 2.021 2.423 2.704 2.9713.551 0.679 .671 2.000 2.390 2.660 2.915 3.460 0.6771.658 1.980 2.358 2.617 2.860 3.373 0.674 1.645 1.960 2.326 2.576 2.807 3.291 2 841 7.453 12.924 4 6 7 8 9 10 15 20 25 30 40 60 120 3.581 3.252 4.073 4.587 NOTE: In calculating confidence intervals, may be substituted for s in Equation 4-6 if you have a great deal of experience with a particular method and have therefore determined its "true" population standard deviation. If is used instead of s, the value oft to use in Equation 4-6 comes from the bottom row of Table 4-2. Table 4-2 Quantitative Chemical Analysis, Seventh Edition © 2007 W. H. Freeman and Company

Explanation / Answer

The desired level of confidence is set by the researcher (not determined by data). Most commonly, the 95% confidence level is used. However, other confidence levels can be used, for example, 90% and 99%.

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