Which of the following statements is true of a house effect? A house effect can be exhibited by nonpartisan firms. A house effect is a result of inadequate knowledge about party candidates. A major cause of a house effect is the absence of party allegiance. A house effect occurs when there is a shortage of opinion leaders.

**Step 1. Define House Effect** A house effect refers to a systematic bias that arises from a particular polling firm’s method of collecting, weighting, or interpreting survey data. It is not inherently tied to partisanship and can be present even in nonpartisan institutions. **Step 2. Evaluate the Statements** 1. **"A house effect can be exhibited by nonpartisan firms."** Since the house effect is about the methodology of polling firms, even nonpartisan firms can exhibit a systematic bias due to their specific techniques or sampling strategies. Thus, this statement is true. 2. **"A house effect is a result of inadequate knowledge about party candidates."** The house effect does not stem from voters' or pollsters’ lack of knowledge about candidates; rather, it is due to methodological issues. This statement is false. 3. **"A major cause of a house effect is the absence of party allegiance."** The absence of party allegiance does not lead to a systematic bias in polling; the house effect is linked to polling techniques, not to voters' political loyalties. This statement is false. 4. **"A house effect occurs when there is a shortage of opinion leaders."** The presence or absence of opinion leaders is unrelated to the methodological biases that create a house effect. This statement is false. **Step 3. Conclusion** The only statement that accurately describes a house effect is: \[ \text{"A house effect can be exhibited by nonpartisan firms."} \]