Understanding the impact of a search system’s response latency on its users’ searching behaviour has been recently an active research topic in the information retrieval and human-computer interaction areas. Along the same line, this paper focuses on the user impact of search latency and makes the following two contributions. First, through a controlled experiment, we reveal the physiological effects of response latency on users and show that these effects are present even at small increases in response latency. We compare these effects with the information gathered from self-reports and show that they capture the nuanced attentional and emotional reactions to latency much better. Second, we carry out a large-scale analysis using a web search query log obtained from Yahoo to understand the change in the way users engage with a web search engine under varying levels of increasing response latency. In particular, we analyse the change in the click behaviour of users when they are subject to increasing response latency and reveal significant behavioural differences.

1. Unconscious Physiological Effects of Search
Latency on Users and Their Click Behaviour
Miguel Barreda-Ángeles (Eurecat), Ioannis Arapakis (Yahoo Labs), Xiao Bai (Yahoo Labs)
B. Barla Cambazoglu (Yahoo Labs), Alexandre Pereda-Baños (Eurecat)

2. Introduction
§ The core research in IR has been on improving the efficiency of
search systems with the eventual goal of satisfying the
information needs of users
§ Most research in this direction had a very system-oriented
viewpoint
§ The impact of efficiency improvements on users’ searching
behaviour and experience have been left unexplored

3. Human Information Processing
§ We are not consciously aware of the
mental processes determining our
behaviour
§ Such unconscious influences reach
from basic or low-level mental
processes to high-level psychological
processes like motivations,
preferences, or complex behaviours

4. Human Information Processing

5. Web Search Latency
§ Previous research in the context of web search has shown that
response latency values lower than a certain threshold are
unnoticeable by the users
§ Conclusions are based on self-report methods which are
inherently limited, since users cannot provide information that is
not consciously available to them
§ We cannot dismiss completely the possibility that even small
latency increases can affect the web search experience

6. Study Focus
§ Impact of response latency increase on user behaviour in web
search
§ Smaller latency values (≥1000ms) that may not be consciously
perceived by users
§ We employ two different yet complementary approaches:
• a small-scale controlled user study
• a large scale query log analysis

7. Controlled User Study

8. Experimental Design
§ Repeated-measures design
§ One independent variable
• search latency* (with four levels in milliseconds: 0, 500, 750, and 1,000)
§ 19 participants (female = 2, male = 17)
§ Dependent variables:
• experienced positive and negative affect
• level of focused attention
• perceived system usability
• participants’ physiological responses
* Search latency was adjusted by a desired amount using a custom-made JS deployed using Greasemonkey.

9. Procedure
§ Participants performed four search tasks
• evaluate the performance of four different backend search systems
• submit as many navigational queries from a list of 200 randomly sampled
web domains
• for each query they were asked to locate the target URL among the first ten
results of the SERP
§ Training queries were used to allow participants to familiarize
themselves with the “default” search site speed

10. Psychophysiological Measures of Engagement
§ User Engagement Scale (UES)
• Positive affect (PAS)
• Negative affect (NAS)
• Perceived usability
• Felt involvement and focused attention
§ IBM’s Computer System Usability Questionnaire
(CSUQ)
• System usefulness (SYSUSE)
§ Electrodermal activity (EDA)
§ Electromyography [corrugator supercilii] (EMG-CS)

11. Characteristics of Psychological Methods
§ Helpful in unveiling attentional and emotional reactions not
consciously available to us
§ Offer high temporal and spatial resolution
§ Robust against cognitive biases (e.g., social desirability bias*)
§ Always provide “honest” responses
§ No direct question to the subject, no direct answer
§ The information on the research questions has to be inferred
from the variations on the physiological signals and the way they
are related to psychological constructs
* The tendency of survey respondents to answer questions in a manner that will be viewed favorably by others.

12. Physiological Data
§ Mixed multilevel models (a regression-based approach)
• allows comparison of data at different levels
• Level 1: conditions within-subjects
• Level 2: subjects
• allows including random terms in the model for random factors
• random intercepts for between-subject variability; accounts for the difference in means between
subjects
• useful for physiological data, since between subject variability can be much larger than variability
due to experimental conditions, and, therefore, can mask it
• random slopes for the effects of time and order of presentation
• Deals with autocorrelated data (e.g. physiological data)

13. EDA Signal
§ Applied 200ms smoothing filter & artifact removal
§ A temporal series was constructed from each physiological signal
§ Averaged the data every 1-second period (480 points == ~ 8 minutes)
§ Each 10-second period following a query submission was visually
inspected for SCRs (skin conductance responses)
§ Data sample: 132 SCRs; 10 points (seconds) by SCR
15.0
15.2
15.4
15.6
15.8
16.0
16.2
16.4
16.6
16.8
17.0
0 1 2 3 4 5 6 7 8 9 10 11 12
µS
Time after stimulus onset (in seconds)

14. EDA Signal
§ Factors considered in the model:
§ random intercept for participants
§ random slope for time and order of presentation
§ fixed factors:
§ latency (4 conditions)
§ seconds (10 seconds)

15. EDA Results
§ Significant increases in the values of EDA through SCRs associated
to the three latency conditions
§ This can be interpreted that, when there is an SCR response, it is
more intense in the three latency conditions (250ms, 500ms and
1000ms) compared to the 0ms condition, i.e., the arousal is higher
for those conditions compared to the 0ms condition
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1 2 3 4 5 6 7 8 9 10
µS
Time after query onset (in seconds)
0ms
500ms
750ms
1000ms
EDA Model
Fixed factors Coefficients
Intercept - .31*
Latency 500ms .50***
Latency 750ms .42**
Latency 1000ms .60***
Seg 2 .11***
Seg 3 .36***
Seg 4 .68***
Seg 5 .88***
Seg 6 .90***
Seg 7 .80***
Seg 8 .74***
Seg 9 .72***
Seg 10 .69***

16. EMG-CS Signal
§ Band-pass filter 30-500Hz & artifact removal
§ A temporal series was constructed from each physiological
signal
§ Averaged the data every 1-second period (480 points == ~ 8
minutes)
§ Included the data for the entire 3-second period after each query
submission
§ Outliers excluded. Data sample: 7256 samples (4 seconds by
query)

17. EMG-CS Signal
§ Factors considered in the model:
§ random intercept for participants
§ random slope for time and order of presentation
§ fixed factors:
§ latency (4 conditions)
§ seconds (10 seconds)

18. EMG-CS Results
§ Significant increases in the values of EMG
associated to the three latency conditions
§ Since EMG over corrugator supercilii is related
to the negative valence of the emotions, the
three latency conditions produced a more
negative valence compared to the 0s latency
condition.
EDA Model
Fixed factors Coefficients
Intercept .0188***
Latency 500ms .0019***
Latency 750ms .0034***
Latency 1000ms .0010*
Seg 1 .0000393
Seg 2 .0002397***
Seg 3 .0003163***

19. Self-Reported Measures of Engagement
Descriptive Statistics (M ± SD) for UE and SYSUSE
0 500ms 750ms 1000ms
Post-Task Positive Affect 17.21 ± 1.46 18.21 ± 1.57 18.68 ± 1.51 17.53 ± 1.79
Post-Task Negative Affect 6.42 ± 0.41 6.32 ± 0.54 6.47 ± 0.48 5.95 ± 0.36
Focused Attention 29.32 ± 1.71 28.26 ± 2.29 27.95 ± 2.18 26.32 ± 2.00
SYSUS 28.16 ± 1.91 29.37 ± 2.09 27.63 ± 1.81 29.05 ± 1.86
24
25
26
27
28
29
30
31
32
0 250 500 750 1000
Meanscores
Latency (in milliseconds)

20. Entropy Analysis
§ We compute two entropy-based features for the EDA and EMG-
CS data:
• Shannon entropy
• Permutation entropy
§ Entropy has been extensively used in signal processing and pattern
recognition
§ In information theory, entropy measures the disorder or uncertainty
associated with a discrete, random variable, i.e., the expected value of
the information in a message

21. Query Log Analysis

22. Setup
§ Random sample of 30m web search queries obtained from Yahoo
Search (issued by approximately 6m users)
§ Each age group involved at least 100K users
§ Similar number of female and male users
§ To control for differences due to geolocation or device, we select
queries issued:
• within the US
• to a particular search data center
• from desktop computers

23. Latency measurement
§ We use the end-to-end (user perceived) latency values
§ We quantify engagement using the clicked page ratio metric
User
Search
frontend
Search
backend
tpre tproc
tpost
tfb
tbf
tuf
tfu
trender

24. Engagement metrics
§ We compare the presence of clicks for two given query instances
(qfast, qslow) that are:
• submitted by the same user
• having the same query string
• matching the same search results
§ Click presence (click-on-fast, click-on-slow)
§ Click count (click-more-on-fast, click-more-on-slow)

25. Results
0
0.05
0.10
0.15
0.20
0 500 750 1000
0
0,5
1.0
1.5
2.0
Fractionofquerypairs
Click-on-fast/Click-on-slow
Latency difference (in milliseconds)
Click-on-fast
Click-on-slow
Ratio
Fig. 1: Fast or slow query response preference according to the click presence
metric.

26. Results
0
0.05
0.10
0.15
0.20
0 500 750 1000
0
0.5
1.0
1.5
2.0
Fractionofquerypairs
Click-more-on-fast/Click-more-on-slow
Latency difference (in milliseconds)
Click-more-on-fast
Click-more-on-slow
Ratio
Fig. 2: Fast or slow query response preference according to the click count metric.

27. Conclusions
§ As the response latency of the search engine reaches higher
values, the arousal and the negative valence of the experienced
emotions increase as well
§ Physiological data showed that the three latency conditions were
associated to:
• higher arousal (SCR data)
• higher negative valence (EMG-CS data)
§ This can be interpreted as a more emotional and negative
experience: a worse experience

28. Conclusions
§ Although the latency effects did not produce changes on the
self-reported data, their impact on users’ physiological
responses is evident
§ Even if such short latency increases of under 500ms are not
consciously perceived, they have sizeable physiological
effects that can contribute to the overall user experience

29. Conclusions
§ A large-scale query log analysis ascertained the effect on the
clicking behaviour of users and revealed a significant decrease
in users’ engagement with the search result page, even at
small increases in latency
§ This highlights the need for a more inter-disciplinary approach
to the evaluation of human information processing in HCI
research

30. Thank you for your attention!
iarapakis
http://www.slideshare.net/iarapakis/sigir15