A key characteristic of informal reviews is low cost. Informal reviews are a type of review that does not follow a formal process or have any formal documentation. Informal reviews are usually performed by individuals or small groups of peers or colleagues who have some knowledge or interest in the product under review. Informal reviews can be done at any time and for any purpose, such as checking for errors, clarifying doubts, sharing ideas, etc. Informal reviews have low cost, as they do not require much time, effort, or resources to conduct. The other options are not key characteristics of informal reviews. Kick-off meeting is a characteristic of formal reviews, such as inspections or walkthroughs. Kick-off meeting is a meeting that is held before the review process starts, where the roles and responsibilities of the participants are defined, the objectives and scope of the review are agreed, and the logistics and schedule of the review are planned. Individual preparation is a characteristic of formal reviews, such as inspections or walkthroughs. Individual preparation is an activity that is performed by the reviewers before the review meeting, where they examine the product under review and identify any issues or questions that need to be discussed or resolved during the review meeting. Metrics analysis is a characteristic of formal reviews, such as inspections or walkthroughs. Metrics analysis is an activity that is performed after the review process is completed, where the data and results of the review are collected and analyzed to measure the effectiveness and efficiency of the review, as well as to identify any improvement actions or lessons learned for future reviews. Verified References: A Study Guide to the ISTQB® Foundation Level 2018 Syllabus - Springer, page 9.