# According to the article “Modeling and Predicting the Effects of Submerged Arc Weldment Process Parameters on Weldment

According to the article “Modeling and Predicting the Effects of Submerged Arc Weldment Process Parameters on Weldment Characteristics and Shape Profiles” (J. of Engr. Manuf., 2012: 1230–1240), the submerged arc welding (SAW) process is commonly used for joining thick plates and pipes. The heat affected zone (HAZ), a band created within the base metal during welding, was of particular interest to the investigators. Here are observations on depth (mm) of the HAZ both when the current setting was high and when it was lower. $$\begin{matrix} Non-high & 1.04 & 1.15 & 1.23 & 1.69 & 1.92 & 1.98 & 2.36 & 2.49 & 2.72 & 1.37 & 1.43 & 1.57 & 1.71 & 1.94 & 2.06 & 2.55 & 2.64 & 2.82 \\ High & 1.55 & 2.02 & 2.02 & 2.05 & 2.35 & 2.57 & 2.93 & 2.94 & 2.97 \\ \end{matrix}$$ c. Does it appear that true average HAZ depth is larger for the higher current condition than for the lower condition? Carry out a test of appropriate hypotheses using a significance level of .01.

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Step 1

Given: $$\displaystyle{n}_{{{1}}}={18}$$
$$\displaystyle{n}_{{{2}}}={9}$$
$$\displaystyle\alpha={0.01}$$ The mean is the sum of all values divided by the number of values: $$\displaystyle\overline{{{x}}}_{{{1}}}={\frac{{{1.04}+{1.15}+{1.23}+\ldots+{2.55}+{2.64}+{2.82}}}{{{18}}}}\approx{1.9261}$$
$$\displaystyle\overline{{{x}}}_{{{2}}}={\frac{{{1.55}+{2.02}+{2.02}+\ldots+{2.93}+{2.94}+{2.97}}}{{{9}}}}\approx{2.3778}$$ The variance is the sum of squared deviations from the mean divided by $$\displaystyle{n}-{1}$$. The standard deviation is the square root of the variance: $$\displaystyle{s}_{{{1}}}=\sqrt{{{\frac{{{\left({1.04}-{1.9261}\right)}^{{{2}}}+\ldots.+{\left({2.82}-{1.9261}\right)}^{{{2}}}}}{{{18}-{1}}}}}}\approx{0.5694}$$
$$\displaystyle{s}_{{{2}}}=\sqrt{{{\frac{{{\left({1.55}-{2.3778}\right)}^{{{2}}}+\ldots.+{\left({2.97}-{2.3778}\right)}^{{{2}}}}}{{{9}-{1}}}}}}\approx{0.5072}$$

Given claim: larger the higher current condition. The claim is either the null hypothesis or the alternative hypothesis. The null hypothesis and the alternative hypothesis state the opposite of each other. The null hypothesis needs to contain the value mentioned in the claim. $$\displaystyle{H}_{{0}}:\mu_{{{1}}}={u}_{{{2}}}$$
$$\displaystyle{H}_{{\alpha}}:\mu_{{{1}}}{<}\mu_{{{2}}}$$

Step 2

Determine the test statistic: $$\displaystyle{t}={\frac{{\overline{{{x}}}_{{{1}}}-\overline{{{x}}}_{{{2}}}}}{{\sqrt{{{\frac{{{{s}_{{{1}}}^{{{2}}}}}}{{{n}_{{{1}}}}}}+{\frac{{{{s}_{{{2}}}^{{{2}}}}}}{{{n}_{{{2}}}}}}}}}}}={\frac{{{1.9261}-{2.3778}}}{{\sqrt{{{\frac{{{0.5694}^{{{2}}}}}{{{18}}}}+{\frac{{{0.5072}^{{{2}}}}}{{{9}}}}}}}}}\approx-{2.093}$$

Determine the degrees of freedom (rounded down to the nearest integer): $$\displaystyle\triangle={\frac{{{\left({\frac{{{{s}_{{{1}}}^{{{2}}}}}}{{{n}_{{{1}}}}}}+{\frac{{{{s}_{{{2}}}^{{{2}}}}}}{{{n}_{{{2}}}}}}\right)}}}{{{\frac{{{\left(\frac{{{s}_{{{1}}}^{{{2}}}}}{{n}_{{{1}}}}\right)}^{{{2}}}}}{{{n}_{{{1}}}-{1}}}}+{\frac{{{\left(\frac{{{s}_{{{2}}}^{{{2}}}}}{{n}_{{{2}}}}\right)}^{{{2}}}}}{{{n}_{{{2}}}-{1}}}}}}}={\frac{{{\left({\frac{{{0.5694}^{{{2}}}}}{{{18}}}}+{\frac{{{0.5072}^{{{2}}}}}{{{9}}}}\right)}^{{{2}}}}}{{{\frac{{{\left(\frac{{0.5694}^{{{2}}}}{{18}}\right)}^{{{2}}}}}{{{18}-{1}}}}+{\frac{{{\left(\frac{{0.5072}^{{{2}}}}{{9}}\right)}^{{{2}}}}}{{{9}-{1}}}}}}}\approx{17}$$

The P-value is the probability of obtaining the value of the test statistic, or a value more extreme. The P-value is the number (or interval) in the column title of Student's T distribution in the appendix containing the t-value in the $$\displaystyle{d}{f}={17}$$: $$\displaystyle{0.025}{<}{P}{<}{0.05}$$

If the P-value is less than or equal to the significance level, then the null hypothesis is rejected: $$\displaystyle{P}{>}{0.01}\Rightarrow$$ Fail to reject $$\displaystyle{H}_{{{0}}}$$ There is not sufficient evidence to support the claim that the true average HAZ depth is larger for the higher current condition than for the lower condition.