Data from: Measurement artefacts lead to false positives in the study of birdsong in noise

1: Numerous studies over the past decade have reported correlations between elevated levels of anthropogenic noise and a rise in the minimum frequency of acoustic signals of animals living in noisy habitats. This pattern appears to be occurring globally, and higher pitched signals have been hypothesized to be adaptive changes that reduce masking by low-frequency traffic noise. However, the sound analysis methods most often used in these studies are prone to measurement errors that can result in false positives. In addition, the commonly used method of measuring frequencies visually from spectrograms might also lead to observer-expectancy biases that could exacerbate measurement errors.
2: We conducted an experiment to (a) quantify the size and type of errors that result from “eye-balling” frequency measurements with cursors placed manually on spectrograms of signals recorded in noise and no-noise conditions, and (b) to test whether observer expectations lead to significant errors in frequency measurements. We asked 54 volunteers, blind to the true intention of our study, to visually measure the minimum frequency of a variety of natural and synthesized bird sounds, recorded either in noise, or no-noise conditions. Test subjects were either informed or uninformed about the hypothesized results of the measurements.
3: Our results demonstrate that inappropriate methodology in acoustic analysis can yield false positives with effect sizes as large, or even larger, than those reported in published studies. In addition to these measurement artefacts, psychological observer biases also led to false positives – when observers expected signals to have higher minimum frequencies in noise, they measured significantly higher minimum frequencies than uninformed observers, who had not been primed with any expectation.
4: The use of improper analysis methods in bioacoustics can lead to the publication of spurious results. We discuss alternative methods that yield unbiased frequency measures and we caution that it is imperative for researchers to familiarize themselves both with the functions and limitations of their sound analysis programs.. In addition, observer expectancy biases are a potential source of error not only in the field of bioacoustics, but in any situation where measurements can be influenced by human subjectivity.

1: 近十年来，诸多研究均报道了人为活动噪声（anthropogenic noise）水平升高，与栖息于噪声生境中的动物声学信号最低频率上升之间存在相关性。该现象似乎在全球范围内普遍存在，学界曾提出假设：更高频的信号是一种适应性演化改变，可降低低频交通噪声对信号的掩蔽效应。然而，此类研究中最常采用的声学分析方法极易产生测量误差，甚至可能引发假阳性结果。此外，通过目视声谱图（spectrogram）手动读取频率的常规方法，还可能引入观察者期望偏差（observer-expectancy bias），进一步加剧测量误差。
2: 本研究开展一项实验，旨在（a）量化通过手动在噪声与无噪声条件下录制的信号声谱图上用光标“目测”频率所产生的误差规模与类型；（b）验证观察者期望是否会导致频率测量出现显著误差。我们邀请54名对研究真实意图不知情的志愿者，对分别录制于噪声与无噪声生境中的多种自然及合成鸟类鸣声的最低频率进行目视测量。实验受试者被分为两组，一组被告知测量的预期结果，另一组则未获得相关提示。
3: 本研究结果显示，声学分析中方法不当可产生假阳性结果，其效应量甚至可与已发表研究中报道的效应量相当，乃至更大。除上述测量伪差外，观察者的心理偏差同样会引发假阳性结果：当观察者预期噪声生境中的信号最低频率更高时，其测得的最低频率会显著高于未被预设任何期望的对照组受试者。
4: 生物声学（bioacoustics）领域中，若采用不当的分析方法，可能导致虚假研究结果得以发表。本文探讨了可生成无偏频率测量值的替代分析方法，并提醒研究者务必充分熟悉其所使用的声学分析软件的功能与局限性。此外，观察者期望偏差不仅是生物声学领域的潜在误差来源，在所有受人类主观性影响的测量场景中，均可能成为误差隐患。