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From page 10... ... 10 Examples of Effective Experiment Design and Data Analysis in Transportation Research About this Chapter This chapter provides a wide variety of examples of research questions. The examples demonstrate varying levels of detail with regard to experiment designs and the statistical analyses required. Read the entire page →
From page 11... ... examples of effective experiment Design and Data analysis in transportation research 11 a comparison of means indicates whether the mean values of two distributions can be considered to be equal with a specified degree of confidence) as well as an operational perspective (e.g., judging whether the difference is large enough to make an operational difference) Read the entire page →
From page 12... ... 12 effective experiment Design and Data analysis in transportation research Areas Covered in the Examples Transportation research is very broad, encompassing many fields. Based on consultation with many research-oriented professionals and a survey of practitioners, key areas of research were identified. Read the entire page →
From page 13... ... examples of effective experiment Design and Data analysis in transportation research 13 Example Area Method of Analysis 1 Structures/bridges Descriptive statistics (exploring and presenting data to describe existing conditions) 2 Public transport Descriptive statistics (organizing and presenting data to describe a system or component) Read the entire page →
From page 14... ... 14 effective experiment Design and Data analysis in transportation research 3. Data Collection: Data are collected at 100 scheduled locations by bridge inspectors. Read the entire page →
From page 15... ... examples of effective experiment Design and Data analysis in transportation research 15 of 4 or below indicates deficiency in a structural component. For the purpose of comparison, the mean bridge condition rating of the 13 structurally deficient bridges also is provided. Read the entire page →
From page 16... ... 16 effective experiment Design and Data analysis in transportation research • Traffic Operations/Safety -- to examine the frequency of turning violations at driveways with various turning restrictions. • Work Zones, Environment -- to assess the average energy consumption during various stages of construction. Read the entire page →
From page 17... ... examples of effective experiment Design and Data analysis in transportation research 17 Bus Route 2008 Revenue 2009 Revenue Local Route 1 $350,500 $365,700 Local Route 2 $263,000 $271,500 Local Route 3 $450,800 $460,700 Local Route 4 $294,300 $306,400 Local Route 5 $173,900 $184,600 Local Route 6 $367,800 $375,100 Local Route 7 $415,800 $430,300 Local Route 8 $145,600 $149,100 Local Route 9 $248,200 $260,800 Local Route 10 $310,400 $318,300 Local Route 11 $444,300 $459,200 Local Route 12 $208,400 $205,600 Local Route 13 $407,600 $412,400 Local Route 14 $161,500 $169,300 Local Route 15 $325,100 $340,200 Express Route 1 $85,400 $83,600 Express Route 2 $110,300 $109,200 Express Route 3 $65,800 $66,200 Express Route 4 $125,300 $127,600 Express Route 5 $90,800 $90,400 Express Route 6 $125,800 $123,400 Express Route 7 $87,200 $86,900 Express Route 8 $68.300 $67,200 Express Route 9 $110,100 $112,300 Express Route 10 $73,200 $72,100 Demand-Responsive System $510,100 $521,300 Table 5. Revenue by route or type of service and year. Read the entire page →
From page 18... ... 18 effective experiment Design and Data analysis in transportation research Express Buses, 15.7% Express Buses, 15.2% Local Buses, 76.3% Local Buses, 75.8% Demand Responsive, 8.5% Demand Responsive, 8.5% 2008 2009 Figure 5. Pie charts illustrating percent of revenue from each component of a transit system. Read the entire page →
From page 19... ... examples of effective experiment Design and Data analysis in transportation research 19 also is based on the assumption that no other factors changed between 2008 and 2009 that might have affected total revenues. One of the implicit assumptions is that the number of riders remained relatively constant from 1 year to the next. Read the entire page →
From page 20... ... 20 effective experiment Design and Data analysis in transportation research 2. Identification and Description of Variables: The variable to be analyzed is the 8-hour average ozone concentration in parts per million (ppm) Read the entire page →
From page 21... ... examples of effective experiment Design and Data analysis in transportation research 21 the test period selected. The mean value of any data set (x _ ) Read the entire page →
From page 22... ... 22 effective experiment Design and Data analysis in transportation research Using the same equation, the mean value for each air-sampling location can be found by summing the value of the ozone concentration in the column representing that location and dividing by the 21 observations at that location. For example, considering Sample Station 1, the mean value of the ozone concentration is 1.709/21 = 0.081 ppm. Read the entire page →
From page 23... ... examples of effective experiment Design and Data analysis in transportation research 23 value. It must be noted that, as presented, this example is not a statistical comparison per se (i.e., there has been no significance testing or formal statistical comparison) Read the entire page →
From page 24... ... 24 effective experiment Design and Data analysis in transportation research variation in the data by day.) The data have a familiar bell shape; that is, when graphed, they look like they are normally distributed (Figure 8) Read the entire page →
From page 25... ... examples of effective experiment Design and Data analysis in transportation research 25 a similar procedure for all speeds. Notice that the areas under the curve can also be calculated in a simple Excel spreadsheet using the "NORMDIST" function for a given x value and the average speed of 10.17 and standard deviation of 2.79. Read the entire page →
From page 26... ... 26 effective experiment Design and Data analysis in transportation research hypothesized distribution. Conversely, smaller variations between observed and expected distributions result in lower values of the statistic, which would suggest that it is more likely that the data are normally distributed because the observed values would fit better with the expected values. Read the entire page →
From page 27... ... examples of effective experiment Design and Data analysis in transportation research 27 The cost of obtaining samples for many construction materials and practices is quite high. As a result, decisions often must be made based on a small number of samples. Read the entire page →
From page 28... ... 28 effective experiment Design and Data analysis in transportation research and using the equation for t above, t = − = = 96 75 95 00 2 63 2 1 75 1 32 1 33 . Read the entire page →
From page 29... ... examples of effective experiment Design and Data analysis in transportation research 29 If there is a severe penalty for missing the deadline (or a significant reward for finishing early) , the contractor may be willing to incur the cost of bringing in a new roller rather than accepting a 15% probability of being delayed. Read the entire page →
From page 30... ... 30 effective experiment Design and Data analysis in transportation research compacted air voids are shown in Table 9. Twenty samples are prepared using the same asphalt mixture. Read the entire page →
From page 31... ... examples of effective experiment Design and Data analysis in transportation research 31 For n - 1 (10 - 1 = 9) degrees of freedom and a = 0.05, the tcrit value can be looked up using a t-table (e.g., NCHRP Project 20-45, Volume 2, Appendix C, Table C-4) Read the entire page →
From page 32... ... 32 effective experiment Design and Data analysis in transportation research of two sets of tests: one from the contractor and one from the agency. Formally, the (null) Read the entire page →
From page 33... ... examples of effective experiment Design and Data analysis in transportation research 33 In this example, a normal distribution is assumed. (The assumption of normality also can be tested, as in Example 4.) Read the entire page →
From page 34... ... 34 effective experiment Design and Data analysis in transportation research Step 3. Determine Fcrit from the F-distribution table, making sure to use the correct degrees of freedom (df) Read the entire page →
From page 35... ... examples of effective experiment Design and Data analysis in transportation research 35 7. Applications in Other Areas of Transportation Research: The simple t-test can be used to compare means of two independent samples. Read the entire page →
From page 36... ... 36 effective experiment Design and Data analysis in transportation research three. (For a discussion on sample size determination based on statistical power requirements, see NCHRP Project 20-45, Volume 2, Chapter 1, "Sample Size Determination.") Read the entire page →
From page 37... ... examples of effective experiment Design and Data analysis in transportation research 37 to be larger than the MSE, such that it will be more likely to reject the null hypothesis. For this example, the calculations for MSB, MSE, and F are as follows: MSB SSB df MSE SSE df F M B E = = = = = 1082878 40 50810 70 . Read the entire page →
From page 38... ... 38 effective experiment Design and Data analysis in transportation research desired. The focus of computations was the construction of the ANOVA table. Read the entire page →
From page 39... ... examples of effective experiment Design and Data analysis in transportation research 39 2. Identification and Description of Variables: The independent consultant runs 12 additional asphalt content tests by taking independent samples from the same pavement section as the agency and contractor. Read the entire page →
From page 40... ... 40 effective experiment Design and Data analysis in transportation research the asphalt content is given in Table 14. (See Example 8 for a more detailed discussion of the calculations necessary to create Table 14.) Read the entire page →
From page 41... ... examples of effective experiment Design and Data analysis in transportation research 41 6. Conclusion and Discussion: This example uses a simple one-way ANOVA to compare the mean values of three sets of results using data drawn from the same test section. Read the entire page →
From page 42... ... 42 effective experiment Design and Data analysis in transportation research 2. Identification and Description of Variables: The engineer identifies 1-mile sections of uniform pavement within the state highway network with similar attributes (aggregate type, slab thickness, joint spacing, traffic, and climate) Read the entire page →
From page 43... ... examples of effective experiment Design and Data analysis in transportation research 43 The data in Table 16 have been produced by considering the original data and following the procedures presented in earlier examples. The emphasis in this example is on understanding what the table of results provides the researcher. Read the entire page →
From page 44... ... 44 effective experiment Design and Data analysis in transportation research populations being compared for their mean difference are not equal or if a proper multiple mean comparisons procedure is not adopted. Based on the comparison of the three means in this example, the engineer can conclude that the pavement slabs having aggregates with a higher CTE value will exhibit more cracking than those with lower CTE values, given that all other variables (e.g., climate effects) Read the entire page →
From page 45... ... examples of effective experiment Design and Data analysis in transportation research 45 dependent (or response) variable in the analysis. Read the entire page →
From page 46... ... 46 effective experiment Design and Data analysis in transportation research Chapter 4. Information about Tukey's procedure can be found in almost any good statistics textbook, such as those by Freund and Wilson [2003] Read the entire page →
From page 47... ... examples of effective experiment Design and Data analysis in transportation research 47 each other when plotted for both factors together, showing no interactive effects between the levels of two factors. Read the entire page →
From page 48... ... 48 effective experiment Design and Data analysis in transportation research then measure the speeds of free-flowing vehicles at the same location after implementing the photo-radar system. Question/Issue Use collected data to determine whether a difference exists between results before and after some treatment is applied. Read the entire page →
From page 49... ... examples of effective experiment Design and Data analysis in transportation research 49 Step 4. Compute the t-statistic using the following equation: t x x S n n n n b a p a b a b = − + Result: t = − ( ) Read the entire page →
From page 50... ... 50 effective experiment Design and Data analysis in transportation research Example 13: Traffic Safety; Complex Before-and-After Comparisons and Controls Area: Traffic safety Method of Analysis: Complex before-and-after comparisons using control groups (examining the effect of some treatment or application with consideration of other factors that may also have an effect) Read the entire page →
From page 51... ... examples of effective experiment Design and Data analysis in transportation research 51 Chapter 1, in the section "Quasi-Experiments." Regression-to-the-mean bias exists when sites with the highest MOE levels in the before time period are treated. If the MOE level falls in the after period, the analyst can never be sure how much of the fall was due to the treatment and how much was due to natural fluctuations in the values of the MOE back toward its usual mean value. Read the entire page →
From page 52... ... 52 effective experiment Design and Data analysis in transportation research 5. Interpreting the Results: Because the calculated chi-square value is greater than the critical chi-square value, the engineer concludes that there is a statistically significant difference in the number of vehicles in the lane to be closed at the midpoint between the before-and-after time periods for the treatment sites relative to what would be expected based on the control sites. Read the entire page →
From page 53... ... examples of effective experiment Design and Data analysis in transportation research 53 2. Identification and Description of Variables: Highway safety, rather the lack of it, is revealed by the total number of fatalities due to motor vehicle crashes. Read the entire page →
From page 54... ... 54 effective experiment Design and Data analysis in transportation research How far might authorities let things go -- 5%? Read the entire page →
From page 55... ... examples of effective experiment Design and Data analysis in transportation research 55 2. Identification and Description of Variables: Bridge inspection generally entails collection of numerous variables including location information, traffic data, structural elements (type and condition) Read the entire page →
From page 56... ... 56 effective experiment Design and Data analysis in transportation research The linear regression model takes the following form: y x ei o i i= + +β β1 where i = 1, 2, . Read the entire page →
From page 57... ... examples of effective experiment Design and Data analysis in transportation research 57 and the model sum of squares (SSR) Read the entire page →
From page 58... ... 58 effective experiment Design and Data analysis in transportation research 0.391% and 0.517% per year. (For a discussion on computing confidence intervals, see NCHRP Project 20-45, Volume 2, Chapter 4.) Read the entire page →
From page 59... ... examples of effective experiment Design and Data analysis in transportation research 59 1. Research Question/Problem Statement: Transportation planners and engineers often work on variations of the classic four-step transportation planning process for estimating travel demand. Read the entire page →
From page 60... ... 60 effective experiment Design and Data analysis in transportation research the analysis process is used to sort through these variables to determine which combination leads to the best model. To be used in ordinary regression modeling, variables need to be continuous; that is, measured ratio or interval scale variables. Read the entire page →
From page 61... ... examples of effective experiment Design and Data analysis in transportation research 61 Restating the now-calibrated model, TRIPS PERS= +4 626 7 515. Read the entire page →
From page 62... ... 62 effective experiment Design and Data analysis in transportation research The model just discussed for evaluating the effect of one independent variable is called a univariate model. Should the final model for this example be multivariate? Read the entire page →
From page 63... ... examples of effective experiment Design and Data analysis in transportation research 63 the dependent variable. This model should reliably explain 33% of the variance of household trip generation. Read the entire page →
From page 64... ... 64 effective experiment Design and Data analysis in transportation research Example 17: Traffic Operations; Regression Analysis Area: Traffic operations Method of Analysis: Regression analysis (developing a model to predict the values that some variable can take as a function of one or more other variables, when not all variables are assumed to be continuous) Read the entire page →
From page 65... ... examples of effective experiment Design and Data analysis in transportation research 65 data points (some sites have provided more than one data point) , covering 15 minutes each, to use in equation (model) Read the entire page →
From page 66... ... 66 effective experiment Design and Data analysis in transportation research Step 5. Finally, the engineer examines the candidate equation for logic and practicality, asking whether the variables make sense, whether the signs of the variables make sense, and whether the variables can be collected easily by design engineers. Read the entire page →
From page 67... ... examples of effective experiment Design and Data analysis in transportation research 67 2. Identification and Description of Variables: Considering a typical, traditional urban area in the United States, it is reasonable to argue that the likelihood of taking public transit to work (Y) Read the entire page →
From page 68... ... 68 effective experiment Design and Data analysis in transportation research An alternative formulation for estimating Pi, the cumulative logistic distribution, is expressed by the following equation: Pi Xi = + − +( ) 1 1 1 2ε β β This function can be plotted as a lazy Z-curve where on the left, with low values of X (low household income) Read the entire page →
From page 69... ... examples of effective experiment Design and Data analysis in transportation research 69 equation, with data for individual households, there is a problem because when Pi happens to equal either 0 or 1 (which is all the time! Read the entire page →
From page 70... ... 70 effective experiment Design and Data analysis in transportation research 6. Conclusion and Discussion: This approach to estimation is not without further problems. Read the entire page →
From page 71... ... examples of effective experiment Design and Data analysis in transportation research 71 2. Identification and Description of Variables: Two types of variables are needed for this analysis. Read the entire page →
From page 72... ... 72 effective experiment Design and Data analysis in transportation research These surveys generally are administered by handing a survey form to people as they enter the transit vehicle and collecting them as people depart the transit vehicle. The surveys also can be administered by mail, telephone, or in a face-to-face interview. Read the entire page →
From page 73... ... examples of effective experiment Design and Data analysis in transportation research 73 all of the survey forms on which these two characteristics (female and over 65 years old) are checked could be extracted and recorded in a table and shown in a frequency diagram. Read the entire page →
From page 74... ... 74 effective experiment Design and Data analysis in transportation research In this example, 1,050 of the 1,850 respondents (57%) have responded that they would use the bus service more frequently if the fare were decreased by $0.xx. Read the entire page →
From page 75... ... examples of effective experiment Design and Data analysis in transportation research 75 alternatives at each test site in terms of numbers of lanes, lane lengths, and such, and then faithfully translate that geometry into links and nodes that the simulation model can use. For each combination of traffic volume and intersection design, the team uses software to find the optimum signal timing and uses that during the simulation. Read the entire page →
From page 76... ... 76 effective experiment Design and Data analysis in transportation research the experimenter has complete control in a simulation, it is common to use efficient designs like factorials and efficient analysis methods like ANOVA to squeeze all possible information out of the effort. Statistical tests comparing the individual mean values of key results by factor are common ways to follow up on ANOVA results. Read the entire page →
From page 77... ... examples of effective experiment Design and Data analysis in transportation research 77 Example 21: Traffic Safety; Non-parametric Methods Area: Traffic safety Method of Analysis: Non-parametric methods (methods used when data do not follow assumed or conventional distributions, such as when comparing median values) Read the entire page →
From page 78... ... 78 effective experiment Design and Data analysis in transportation research Having decided on the Wilcoxon signed-rank test, the engineer arranges the data (see Table 31) Read the entire page →
From page 79... ... examples of effective experiment Design and Data analysis in transportation research 79 the new signs. The engineer has kept the duration short between the before-and-after data collection periods, which helps minimize the chances of other important changes. Read the entire page →
From page 80... ... 80 effective experiment Design and Data analysis in transportation research • Equation fit: See Volume 2, Chapter 4, Glossary, Descriptive Measures of Association Between X and Y • Error distributions (normality, constant variance, uncorrelated, etc.) Read the entire page →
From page 81... ... Abbreviations and acronyms used without deﬁnitions in TRB publications: AAAE American Association of Airport Executives AASHO American Association of State Highway Officials AASHTO American Association of State Highway and Transportation Officials ACI–NA Airports Council International–North America ACRP Airport Cooperative Research Program ADA Americans with Disabilities Act APTA American Public Transportation Association ASCE American Society of Civil Engineers ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials ATA American Trucking Associations CTAA Community Transportation Association of America CTBSSP Commercial Truck and Bus Safety Synthesis Program DHS Department of Homeland Security DOE Department of Energy EPA Environmental Protection Agency FAA Federal Aviation Administration FHWA Federal Highway Administration FMCSA Federal Motor Carrier Safety Administration FRA Federal Railroad Administration FTA Federal Transit Administration HMCRP Hazardous Materials Cooperative Research Program IEEE Institute of Electrical and Electronics Engineers ISTEA Intermodal Surface Transportation Efficiency Act of 1991 ITE Institute of Transportation Engineers NASA National Aeronautics and Space Administration NASAO National Association of State Aviation Officials NCFRP National Cooperative Freight Research Program NCHRP National Cooperative Highway Research Program NHTSA National Highway Traffic Safety Administration NTSB National Transportation Safety Board PHMSA Pipeline and Hazardous Materials Safety Administration RITA Research and Innovative Technology Administration SAE Society of Automotive Engineers SAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (2005) TCRP Transit Cooperative Research Program TEA-21 Transportation Equity Act for the 21st Century (1998) Read the entire page →

From page 10...

... 10 Examples of Effective Experiment Design and Data Analysis in Transportation Research About this Chapter This chapter provides a wide variety of examples of research questions. The examples demonstrate varying levels of detail with regard to experiment designs and the statistical analyses required.