Missing data is a pervasive issue in real-world analytics, stemming from a multitude of factors (e.g., device malfunctions and network disruptions), making it a ubiquitous challenge in many domains. Misperception of missing data impacts decision-making and causes severe consequences. To mitigate risks from missing data and facilitate proper handling, computing methods (e.g., imputation) have been studied, which often culminate in the visual representation of data for analysts to further check. Yet, the influence of these computed representations on user judgment regarding missing data remains unclear. To study potential influencing factors and their impact on user judgment, we conducted a crowdsourcing study. We controlled 4 factors: the distribution, imputation, and visualization of missing data, and the prior knowledge of data. We compared users' estimations of missing data with computed imputations under different combinations of these factors. Our results offer useful guidance for visualizing missing data and their imputations, which informs future studies on developing trustworthy computing methods for visual analysis of missing data.