To find the critical value(s), we need more context, particularly for what type of test you are conducting. Generally, the type of test could be:

1. t-test: If you're comparing the means of two samples (such as comparing the mean scores of 30-second and 60-second ads).
2. z-test: If you're conducting a hypothesis test with a known population variance.
3. chi-square test: For categorical data comparison.
4. ANOVA (F-test): If you're comparing more than two groups.

Given that you have two groups of data (30-second ads and 60-second ads), it's possible you're performing a t-test to compare the means of these two groups.

Here are the steps to find the critical value for a t-test:

1. Determine the degrees of freedom (df): $df = n_1 + n_2 - 2$ where $n_1$ and $n_2$ are the number of observations in the 30-second and 60-second ads groups.

2. Select the significance level (α) for your test. This is typically 0.05 for a 95% confidence level, but the question might specify another value.

3. Use a t-distribution table or calculator to find the critical value based on the degrees of freedom and the significance level.

Let's go step by step:

1. The sample sizes for both groups are:

   • For 30-second ads: $n_1 = 10$
   • For 60-second ads: $n_2 = 10$

   So, the degrees of freedom $df = 10 + 10 - 2 = 18$.

2. If we assume a significance level $\alpha = 0.05$, for a two-tailed test, the critical t-value can be found using a t-distribution table or a calculator for $df = 18$ and $\alpha = 0.05$.

Let me calculate the critical value for you.The critical value for a two-tailed t-test with 18 degrees of freedom and a significance level of $\alpha = 0.05$ is approximately 2.10.

Would you like to proceed with additional calculations, or do you need more details about this? Here are five follow-up questions that might help you expand your understanding:

1. What are the sample means and standard deviations for the two groups (30-second and 60-second ads)?
2. How do you calculate the test statistic for comparing two independent means?
3. What is the p-value, and how does it relate to the critical value in hypothesis testing?
4. How would the critical value change if we used a one-tailed test instead of a two-tailed test?
5. How does increasing the sample size affect the critical value and the outcome of the test?

Tip: When conducting a t-test, always verify whether the variances of the two groups are equal. This determines whether you should use the pooled variance formula.