[Context] Systematic Literature Review (SLR) has been a major type of study published in Software Engineering (SE) venues for about two decades. However, there is a lack of understanding of whether an SLR is really needed in comparison to a more conventional literature review. Very often, SE researchers embark on an SLR with such doubts. We aspire to provide more understanding of when an SLR in SE should be conducted. [Objective] The first step of our investigation was focused on the dataset, i.e., the reviewed papers, in an SLR, which indicates the development of a research topic or area. The objective of this step is to provide a better understanding of the characteristics of the datasets of SLRs in SE. [Method] A research synthesis was conducted on a sample of 170 SLRs published in top-tier SE journals. We extracted and analysed the quantitative attributes of the datasets of these SLRs. [Results] The findings show that the median size of the datasets in our sample is 57 reviewed papers, and the median review period covered is 14 years. The number of reviewed papers and review period have a very weak and non-significant positive correlation. [Conclusions] The results of our study can be used by SE researchers as an indicator or benchmark to understand whether an SLR is conducted at a good time.

翻译：[背景] 近二十年来，系统文献综述（SLR）已成为软件工程（SE）领域发表的主要研究类型之一。然而，与传统文献综述相比，人们对SLR是否真的必要仍缺乏理解。软件工程研究者常带着此类疑问开展SLR。我们旨在为软件工程中何时应开展SLR提供更多认识。[目的] 本研究的首要步骤聚焦于SLR的数据集（即被审阅的论文），该数据集反映了研究主题或领域的发展。此步骤的目标在于更好理解软件工程领域SLR数据集的特征。[方法] 我们对发表于顶级软件工程期刊的170篇SLR样本进行了研究综述。我们提取并分析了这些SLR数据集的量化属性。[结果] 结果表明，样本中数据集规模的中位数为57篇审阅论文，覆盖的审阅周期中位数为14年。审阅论文数量与审阅周期之间呈非常微弱且不显著的正相关。[结论] 本研究结果可作为软件工程研究者的指标或基准，用于判断SLR是否在适当时机开展。